Sixty-nine genera or subgenera of Mesozic Bivalvia hitherto proposed on the materials from Japan and its adjacent areas are reviewed, and such basic information as the reference of original description, revised systematic position, type-species, main diagnostic characters, geologic age and geographic distribution of each taxon is indicated. The systematic position of several taxa seem to have been misinterpreted in previous works, and different views from the Treatise on invertebrate paleontology, part N, if present, are explained in the remarks. The distribution of these genera and subgenera is in most cases restricted to Japan and its adjacent areas, but they may be important for the general consideration of bivalve phylogeny and classification. The diagnoses are in many cases adapted or abridged from original or subsequent emended descriptions (source in parentheses) but, if necessary, are written afresh.

Trigonucula Ichikawa, 1949, Japan. Jour. Geol. Geogr., 21 (1-4): 267.
Systematic position. —A genus of the Nuculidae.
Type-species. —Trigonucula sakawana Ichikawa, 1949 (original designation) [Pl, 1, Fig. 1]
Diagnosis. —Shell roundly trigonal, subequilateral; umbo subcentral, somewhat opisthogyrous; escutcheon delimited by a sharp ridge; ventral margin smooth internally; no radial ornament; hinge typical of nuculid, but two series of denticles forming a more acute angle than in Nucula and many other genera of the Nuculidae; resilifer not so oblique as in Nucula.
Age and distribution. —Carnian. Japan.

Explanation of Plate 1 Fig. 1. Trigonucula sakawana Ichikawa: type-species of genus Trigonucula. Holotype (UMUT MM 5242), rubber cast from left external mould, ×2. Loc. Umenokidani, Sakawa (Carnian, Kochigatani group). Fig. 2. Solemya suprajurensis Hayami. Holotype (UMUT MM3138), internal mould of right valve, ×1. Loc. Mitarai, Makito (Callovian, Tetori group). Fig. 3, Parallelodon (Palaeocucullaea) monobensis Nakazawa: type-species of subgenus Palaeocucullaea. Rubber cast from left internal mould (UMUT MM4571), ×1. Loc. Hirabarazaka, Mine (Carnian, Mine group). Fig. 4. Parallelodon (Torinosucatella) kobayashii (Tamura): type-species of subgenus Torinosucatella. Rubber cast from left external mould (UMUT MM3207), ×1.5. Loc. Tatenosawa, Soma (Kimmeridgian, Soma group). Fig. 5. The same species. Left valve (UMUT MM3206), ×1.5. Loc. ditto. Fig. 6. Matsumotoa japonica Okada: type-species of genus Matsumotoa. Holotype (GK H6027), rubber cast from right internal mould, ×1. Loc. Tsuzumugi, Mifune (Cenomanian-Turonian, Mifune group). Fig. 7. The same species. Paratype (GK H6026), left valve, ×1.5. Loc. Kanayama, Masuki (Cenomanian-Turonian, Mifune group). Fig. 8. Glycymeris (Pseudoveletuceta) mifunensis Tashiro: type-species of subgenus Pseudoveletuceta. Holotype (KE 1774), rubber cast from right internal mould, ×1.5. Loc. Asanoyabu, Mifune (Cenomanian-Turonian, Mifune group). Fig. 9. Glycymeris (Hanaia) densilineata Nagao: type-species of subgenus Hanaia. Left valve (GK H6219), ×1.5. Loc. Haipe, Tanohata (Aptian, Miyako group). Fig. 10. The same species. Topotype (GK H6352), right valve, ×1.5. Loc. Hiraiga, Tanohata (Aptian, Miyako group). Fig. 11. Inoperna plicata (Sowerby). Right valve (UMUT MM3264), ×1. Loc. Yamashita, Soma (Kimmeridgian, Soma group). Fig. 12. Modiolus bakevelloides (Hayami). Holotype (UMUT MM2719), right valve, ×1. Loc. Niranohama, Shizukawa (Hettangian, Shizukawa group). Fig. 13. Pinna (Plesiopinna) airiniformis (Amano): type-species of subgenus Plesiopinna. Holotype (KU not registered), right valve, ×1. Loc. Shishijima (Albian?-Cenomanian, Gosyonoura group). Fig. 14. Pteria masatanii Tamura. Holotype (UMUT MM3236), left valve, ×1. Loc. Tatenosawa, Soma (Kimmeridgian, Soma group). Fig. 15. Pteroperna pauciradiata Tamura. Holotype (UMUT MM3246), left valve, ×1. Loc. Tatenosawa, Soma (Kimmeridgian, Soma group). Fig. 16. Somapteria koikensis Tamura: type-species of genus Somapteria. Holotype (UMUT MM3242), right valve, ×1.5. Loc. Koike, Soma (Kimmeridgian, Soma group).

Ezonuculana Nagao, 1938, Jour. Fac. Sci. Hokkaido Imp. Univ., [4], 4 (1-2): 121.
Systematic position. —A subgenus of Jupiteria (Nuculanidae)
Type-species. —Nuculana mactraeformis Nagao, 1932 (original designation) [Pl. 9, Fig. 1]
Diagnosis. —Shell inflated, short, subtrigonal, not distinctly rostrated posteriorly, with subcentral and slightly opisthogyrous umbo; no lunule; indistinct escutcheon; surface smooth; hinge in two angular rows of nearly equal length; chondrophore distinct and wide; pallial sinus obsolete; test apparently not nacreous. (abridged from Ichikawa and Maeda, 1958b, p. 86)
Age and distribution. —Coniacian-Campanian. Japan and Saghalin.
Remarks. —Nagao originally proposed Essonucula as a subgenus of Nuculana, but, as regarded by Ichikawa and Maeda (1948b), it seems to be more closely related to Jupiteria Bellardi, 1875, which is, however, characterized by more distinctly sinuated pallial line.

Explanation of Plate 9 Fig. 1. Jupiteria (Ezonuculana) mactraeformis (Nagao): type-species of subgenus Ezonuculana. Left internal mould (OCU MM191), × 3. Loc. Kamatani, Izumi mountains (Campanian, Izumi group). Reproduced from Ichikawa and Maeda, 1958b, pl, 5, fig. 2a. Fig. 2. Bakevellia (Yoshimopsis) nagatoensis (Ohta): type-species of sugenus Yoshimopsis. Holotype (GF Y423), left internal mould, ×1. Loc. Yoshimo, Shimonoseki (Neocomian, Yoshimo formation). Reproduced from Ohta, 1974, pl. 1, fig. 11. Fig. 3. Micronectes bellaturus Ichikawa and Maeda: type-species of genus Micronectes. Holotype (OCU MM237), left external mould, × 7. Loc. Kamatani, Izumi mountains (Campanian, Izumi group). Reproduced from Ichikawa and Maeda, 1958b, pl. 5, fig. 15b. Fig. 4. Plicatounio (Plicatounio) naktongensis naktongensis Kobayashi and Suzuki: nominal subspecies of the type-species of genus Plicatounio. Holotype (to be preserved in the Geol. Survey of Korea), left valve of a bivalved specimen, ×1. Loc. Ryohori, Keisyo-nan-do (Low. Cretaceous, Naktong group). Reproduced from Kobayashi and Suzuki, 1936, pl. 28, fig. 1b. Fig. 5. The same species. Right valve of the same specimen. Reproduced from Kobyashi and Suzuki, 1936, pl. 28, fig. 1a. Fig. 6. Paranodonta otai Kobayashi and Suzuki: type-species of genus Paranodonta. Holotype (UMUT MM7921), right valve of a bivalved specimen, ×1. Loc. Rikimaru, Kurate (Kwanmon group). Reproduced from Kobayashi and Suzuki, 1936, pl. 17, fig. 7. Fig. 7. Pleurogrammatodon splendens Ichikawa and Maeda: type-species of genus Pleurogram matodon. Holotype (OCU MM145), side view (7a) and upper view (7b) of a left valve, ×1. Loc. Nada, Awaji (Maestrichtian, Izumi group). Reproduced from Ichikawa and Maeda, 1958a, pl. 1, figs. 1c, 2. Fig. 8. Microtrigonia amanoi Nakano: type-species of genus Microtrigonia. Holotype (KUMA0000la), clay cast of bivalved external mould, × 2. Loc. Ukimizu, Shimokoshiki (Santonian or Campanian, Himenoura group). Reproduced from Nakano, 1957a, pl. 9, fig. 21.

Palaeocucullaea Tokuyama, 1960c, Japan. Jour. Geol. Geogr., 31 (2-4); 205.
Systematic position. —A subgenus of Parallelodon (Parallelodontidae).
Type-species. —Parallelodon monobensis Nakazawa, 1955 (original designation) [Pl. 1, Fig. 3]
Diagnosis. —shell roundly elongate-trapezoidal; ventral sinuation, if present, shallow; posterior margin rounded with indistinct posterior wing; ligament area triangular, broad in adult shells, with several chevrons; teeth radiating from the upper side, composed of a few opisthoclinal anterior teeth, a few prosoclinal posterior teeth and many small median denticles.
Age and distribution. —Carnian-(?) Lower Jurassic. Japan, east Siberia and (?) New Zealand.
Remarks. —palaeocucullaea was synonymized with Parallelodon Meek and Worthen,1866, in the Treatise, but subgeneric distinction from Parallelodon (s. s.) seems to be appropriate in view of the Barbatia-iike rounded outline and the Idonearca-like inclined anterior and posterior teeth.

Torinosucatella Tamura, 1959d, Trans. Proc. Pal. Soc. Japan, n. s., (34): 55.
Systematic position. —A subgenus of Parallelodon (Parallelodontidae).
Type-species. —Catella (Torinosucatella) kobayashii Tamura, 1959 (original designation) [Pl. 1, Figs. 4, 5]
Diagnosis. —Shell small, arciform, posteriorly alate, submesially constricted; ventral margin deeply concave in accordance with the constriction; surface with fine radial ribs and concentric wrinkles; hinge similar to Parallelodon (Cosmetodon).
Age and distribution. —Upper Jurassic. Japan.
Remarks. —Torinosucatella was originally regarded as a subgenus of Catella Healey, 1908, but, as treated by Hayami in Hayami, Sugita and Nagumo (1960), the general outline, ventral sulcus and surface ornamentation indicate that it is more closely related to Parallelodon, especially to its subgenus Cosmetodon. The figure of "Catella (Torinosucatella) kobayashii" in the Treatise (p. N259, figs. C7-1a, b) was erroneously reproduced from Tamura's (1959d) illustration of Catella (Catella) laticlava Healey.

Pleurogrammatodon Ichikawa and Maeda, 1958a, Jour. Inst. Polytech. Osaka City Univ., [G],3:63.
Systematic position. —A genus of the Parallelodontidae.
Type-species. —Pleurogrammatodon splendens Ichikawa and Maeada, 1958 (original disignation) [Pl. 9, Figs. 7a, b]
Diagnosis. —Shell large, elongate, thick-shelled, with improminent umbo and wide triangular limament area; no ventral sinus; surface with strong radial ribs which are dissimilar between two valves as in Indogrammatodon; dentition consisting of three or more elongated posterior teeth, a series of subvertical median denticles and three or four short anterior teeth, (abridged from Ichikawa and Maeda, 1958a, p. 63)
Age and distribution. —Campanian-Maestrichtian. Japan, Madagascar and (?) west Canada.
Remarks. —In the Treatise Pleurogrammatodon was assigned as a subgenus of Nemodon Conrad, 1869, but many essential characters seem to indicate that it is more closely related to Indogrammatodon Cox, 1937. Here I agree with Ichikawa and Maeda (1958a) in considering that Pleurogrammatodon is a distinct genus of the Grammatodontinae. The figure of "Nemodon (Pleurogrammatodon) splendens" in the Treatise (p. N259, fig. C7-5a, only) does not agree with Ichikawa and Maeda's illustration and any other specimens of this type-species, but was probably erroneously reproduced from some figure of an unrelated bivalve.

Matsumotoa Okada, 1958, Mem. Fac. Sci. Kyushu Univ., [D], 8 (2): 36.
Systematic position. —A genus of the Noetiidae.
Type-species. —Matsumotoa japonica Okada, 1958 (original designation) [Pl. 1, Figs. 6, 7].
Diagnosis. —Shell ovally subtapezoidal, sometimes transversely elongated; umbo orthogyrous or slightly prosogyrous, located at about the anterior third; large submedian depression running from umbonal area to ventral margin; ventral margin broadly sinuate and smooth internally; ligament area subtriangular, vertically striated without chevrons; hinge plate broad, with ventrally converging short median denticles and a number of slender and unusually long teeth on anterior and posterior sides; surface with several narrow radial riblets.
Age and distribution. —Cenomanian-Turonian. Japan.

Hanaia Hayami, 1965a, Mem. Fac. Sci. Kyushu Univ., [D], 15 (2): 250.
Systematic position. —A subgenus of Glycymeris (Glycymerididae),
Type-species. —Glycymeris densilineata Nagao, 1934 (original designation) [Pl. 1, Figs. 9,10]
Diagnosis. —Shell very small, strongly inflated, prosocline; umbo submesial; inner ventral margin coarsely crenulated; hinge with stout and elongated lateral teeth and not much degenerated median denticles; flattened area along periphery of hinge plate un- usually narrow; surface smooth except for broad rounded radial ribs and grooves on which there are numerous fine radial striae. (abridged from Hayami, 1965a, p. 250)
Age and distribution. —Aptian-Albian. Japan.
Remarks. —Hanaia is regarded as a small and primitive group of Glycymeris, characterized by strong convexity, coarse ventral crenulation and comparatively large teeth extending near the periphery of hinge plate. This was not cited in the Treatise. Tashiro (1971) referred some Late Cretaceous species of Glycymeris from Japan to this subgenus, but I think they are more appropriately referable to the subgenus Glycymerita Finlay and Marwick, 1937.

Pseudoveletuceta Tashiro, 1971, Trans. Proc. Pal. Soc. Japan, n. s., (84): 236.
Systematic position. —A subgenus of Glycymeris (Glycymeridiae).
Type-species. —Glycymeris (Pseudoveletuceta) mifunensis Tashiro, 1971 (original designation) [Pl. 1, Fig. 8]
Diagnosis. —Shell orbicular to ovate, prosocline, highly inequilateral; umbo small, prosogyrous, located very anteriorly; surface smooth except for fine radial threads which do not form bundles; inner ventral margin densely crenulated. (abridged from Tashiro, 1971, p. 236)
Age and distribution. —Cenomanian-Turonian. Japan.
Remarks. —In spite of the Limopsis-like, external features, the presence of ventral crenulations and chevron-shaped ligament indicates that this is a specialized group of Glycymeris.

Plesiopinna Amano, 1956, Kumamoto Jour. Sci., [B], [1], 2 (1): 70.
Systematic position. —A subgenus of Pinna (Pinnidae).
Type-species. —Plesiopinna atriniformis Amano, 1956 (original designation) [Pl. 1, Fig. 13]
Diagnosis. —Shell comparatively small, wedge-shaped; median ridge distinct young shells but later obsolete; posterior margin subvertically truncated and closed; surface with or without radial ribs.
Age and distribution. —Albian-Cenomanian. Japan.
Remarks. —Plesiopinna, which was originally proposed as a distinct genus of the Pinnidae, is here treated as a subgenus of Pinna in agreement with the Treatise. So far as the original specimens are observed, the closed posterior margin is not due to secondary deformation but does characterize this subgenus.

Somapteria Tamura, 1960a, Trans. Proc. Pal. Soc. Japan, n. s., (37): 224.
Systematic position. —A genus of the Pteriidae.
Type-species. —Somapteria koikensis Tamura, 1960 (original designation) [Pl. 1, Fig. 16]
Diagnosis. —Shell small, equivalve, subrhomboidal, obliquely elongated; umbo not prominent; both valves possessing obtusely truncated posterior wing and fairly large pointed anterior wing; surface smooth; hinge composed of a few short anterior teeth and two elongated posterior teeth in each valve; ligament area narrow, of pteriid-type.
Age and distribution. —Kimmeridgian. Japan.

Eopinctada Tamura, 1961, Trans. Proc. Pal. Soc. Japan, n. s., (44); 147.
Systematic position. —A subgenus of Pinctada (Pteriidae).
Type-species. —Pinctada (Eopinctada) matsumotoi Tamura, 1961 (original designation) [Pl. 2, Fig. 2]
Diagnosis. —Shell subequivalve, obliquely trigonal, higher than long, thick, not alate; umbo subterminal, prosogyrous; byssal margin deeply concave; antero-ventral and posterior margins converging to meet at subangular postero-ventral extremity; surface smooth; cardinal area wide, with a large and obliquely triangular pit; hinge edentulous in adult shells; adductor muscle scar crescentic, strongly impressed.
Age and distribution. —Cenomanian-Turonian. Japan.
Remarks. —Although Eopinctada was regarded in the Treatise as generically distinct from Pinctada, the ligament and other essential characters are similar as originally described by Tamura (1961).

Explanation of Plate 2 Fig. 1. Pterinella shinoharai Hayami. Holotype (GK H623S), plaster cast from right internal mould, ×0.67. Loc. Nekodani, Yatsushiro (Albian, Yatsushiro formation). Fig. 2. Pinctada (Eopinctada) matsumotoi Tamura: type-species of subgenus Eopinctada, Holotype (UMUT MM3704), rubber cast from right internal mould, ×1. Loc. Kamiumeki, Mifune (Cenomanian-Turonian, Mifune group). Fig. 3. Bakevellia (Neobakevellia) trigona (Yokoyama). Topotype (UMUT MM2651), left valve, ×1. Loc. Niranohama, Shizukawa (Hettangian, Shizukawa group). Fig. 4. Bakevellia (Bakevelloides) hekiensis (Kobayashi and Ichikawa): type-species of subgenus Bakevelloidcs. Plaster cast from left internal mould (UMUT MM4485), ×1. Loc. Shiraiwa, Mine (Carman, Mine group). Fig. 5. Waagenoperna triangularis (Kobayashi and Ichikawa). Rubber cast from right internal mould (UMUT MM4494), ×2. Loc. Higaeribara, Mine (Carnian-Norian, Mine group). Fig. 6. Kobayashites hemicylindricus Hayami: type-species of genus Kobayashites, Holotype (UMUT MM3130), rubber cast from left internal mould, ×2. Loc. Akaiwazaki, Shizukawa (Bajocian, Hashiura group). Fig. 7. Inoceramus (Inoceramus) maedae Hayami. Holotype (UMUT MM9076), right valve, ×0.86. Loc. Mitarai, Makito (Callovian, Tetori group). Fig. 8. Asoella confertoradiata (Tokuyama): type-species of genus Asoella. Holotype (UMUT MM4498), rubber cast from left external mould, ×1.5. Loc. Higaeribara, Mine (Carnian-Norian, Mine group). Fig. 9. The same species. Rubber cast from left internal mould (UMUT MM4503), ×1.5. Loc. ditto. Fig, 10. Daonella kotoi Mojsisovics. Holotype (UMUT MM5001), clay cast from right external mould, ×1. Loc. Zohoin, Sakawa (Ladinian, Zohoin group).

Neobakevellia Nakazawa, 1959, Mem. Coll. Sci. Univ. Kyoto, [B], 26 (2): 200.
Systematic position. —A subgenus of Bakevellia (Bakevelliidae).
Type-species. —Gervillia costata von Schlotheim, 1820 (original designation).
Diagnosis. —Medium- or large-sized Bakevellia, with much degenerated anterior adductor muscle scar; subequivalve to fairly inequivalve; hinge teeth differentiated into a few short anterior teeth and one or two elongated posterior teeth; ligament pits typically triangular and small in number but subquadrate and numerous in advanced species.
Age and distribution. —Lower Triassic-Upper Cretaceous. Cosmopolitan.
Remarks. —In the Treatise, Neobakevellia was treated as a junior synonym of subgenus Bakevelloides Tokuyama, 1959, notwithstanding that Bakevellia costata (von Schlotheim), he type-species of Neobakevellia, was figured as an example of Bakevellia (Bakevellia). Bakevellia costata and some other Lower-Middle Triassic species are actually not much different from the Permian typical representatives of Bakevellia (s. s) but the degeneration of anterior adductor muscle scar is noteworthy. Upper Triassic and later species except for the species referable to Bakevelloides and Yoshimopsis are generally much larger, highly inequivalve and provided with more numerous ligament pits. So far as various Mesozoic species are observed, such advanced species are at least subgenerically separable from the Permian type-species. The Lower-Middle Triassic species including Bakevellia costata appear to be intermediate in many characters. The subgeneric name of Neobakevellia is tentatively used for the most Mesozoic species of Bakevellia from Japan, though further comparative studies may be needed about the systematic position and affinities of Odontoperna Frech, 1891, and Pseudogervilleia Gillet, 1922, which were similarly proposed for Mesozoic representatives.

Maixuria Nakazawa, 1959, Mem. Coll. Sci. Univ. Kyoto, (B), 26 (2): 201.
Systematic position. —Subjective synonym of Neobakevellia Nakazawa, 1959 (Bakevelliidae).
Type-species. —Bakevellia (Maissuria) kambei Nakazawa, 1959 (original designation).

Bakevelloides Tokuyama, 1959a, Trans. Proc. Pal. Soc. Japan, n. s., (35): 148.
Systematic position. —A subgenus of Bakevellia (Bakevelliidae).
Type-species. —Gervillia hekiensis Kobayashi and Ichikawa, 1952 (original designation) [Pl. 2, Fig. 4]
Diagnosis. —Medium-sized Bakevellia, with thick, subequivalve and roundly triangular shells; ligament area wide, depressed triangular, with a few deep pits; anterior teeth commonly numerous, small, divergent from umbo; posterior wing undeveloped, obtusely or subrectangularly truncated.
Age and distribution. —Middle Triassic-Upper Jurassic. Japan, western Europe and Petchora.
Remarks. —Tokuyama (1959a) regarded Bakevelloides as a distinct genus of the Bakevelliidae. Although the pseudotaxodont denticles are diagnostic, Bakevelloides is similar to Bakevellia (s. s.) in the subequivalve and roundly triangular outline. The taxonoroic treatment in the Treatise, where Bakevelloides was regarded as a subgenus of Bakevellia, seems to be appropriate.

Yoshimopsis Ohta, 1974, Bull. Fukuoka Univ. Educ., 23 (3): 79.
Systematic position. —A subgenus of Bakevellia (Bakevelliidae).
Type-species. —Bakevelloides (Yoshimopsis) nagatoensis Ohta, 1974 (original designation) [Pl. 9, Fig. 2]
Diagnosis. —Medium-sized bakevelliids with subtrapezoidal to subrhomboidal shell and underveloped posterior wing; hinge plate narrow but extending along entire length of dorsal margin; anterior teeth taxodont-like, numerous, diverging from upper side; surface with weak radial riblets in young stage (abridged from Ohta, 1974, p. 79)
Age and distribution. —Neocomian. Japan.
Remarks. —Yoshimopsis, as originally considered, may be related to Bakevelloides, but is here treated as a subgenus of Bakevellia. It is distinguishable from Bakevellia (s. s.) and Bakevelloides by the extremely long hinge area with more numerous denticles and the presence of distinct radial riblets in young shells.

Kobayashites Hayami, 1959e, Trans. Proc. Pal. Soc. Japan, n. s., (35): 138.
Systematic position. —A genus of the Bakevelliidae.
Type-species. —Kobayashites hemicylindricus Hayami, 1959 (original designation) [Pl. 2, Fig. 6]
Diagnosis. —Shell small, soleniform, elongate-rectangular, highly inequivalve, not alate; left valve moderately inflated, hemicylindrical; right valve nearly flat; umbo subterminal, prosogyrous; byssal gape wide; ligament area narrow, with several widely separated pits; hinge of Sakevellia-type, but anterior teeth obliquely elongated, supported by septum-like thickening below umbo; surface smooth.
Age and distribution. —Bajocian. Japan.

Waagenoperna Tokuyama, 1959a, Trans. Proc. Pal. Soc. Japan, n. s., (35): 151.
Systematic position. —A genus of the Isognomonidae.
Type-species. —Edentula lateplanata Waagen, 1907 (original designation)
Diagnosis. —Shell subequivalve, mytiliform or cuneiform, compressed; both wings not clearly delimited; umbo prosogyrous, subterminal; ligament area elongated along dorsal margin but becoming somewhat obscure near posterior end, provided with several subquadrate pits of Isognomon-type; anterior and posterior teeth obsolete already in early stage; no radial ribs.
Age and distribution. —Middle Permian-Upper Triassic. Middle Europe and Japan.
Remarks,-Waagenoperna was proposed as a substitute name for Edentula Waagen, 1907, non Nitzsch, 1820. Cox (1954) established a new genus Cuneigervillia also "as the substitute" for Edentula Waagen, 1907, but the type-species of that genus, Gervillia hagenowi, may belong to the Bakevelliidae and is clearly different from Waagen's species in many characters. Waagenoperna, as interpreted in the Treatise, should be regarded as a distinct genus of the Isognomonidae. Nakazawa and Newell (1968) clarified that Waagenoperna is already well represented in the Middle-Upper Permian bivalve fauna of Japan. Nakazawa and Murata (1966) regarded an early Cretaceous species from north Japan as belonging to this genus, but the generic reference seems to be quite doubtful.

Asoella Tokuyama, 1959b, Japan. Jour. Geol. Geogr., 30: 2.
Systematic position. —A genus of the Aviculopectinidae.
Type-species. —Eumorphotis (Asoella) confertoradiata Tokuyama, 1959 (original designation) [Pl. 2, Figs. 8, 9]
Diagnosis. —Shell small, roundly subquadrate, nearly acline; left valve strongly convex, with broadly inflated umbonal area; right valve weakly convex; right anterior auricle clearly demarcated, projecting forward; byssal notch shallow but distinct; other auricles obtusely or subrectangularly truncated; surface with or without fine radial ribs.
Age and distribution. —Carnian-Norian. Japan.
Remarks. —Asoella was originally proposed as a subgenus of Eumorphotis Bittner, 1901. Although all the materials of the type-species and two other simultaneously described species are poorly preserved, Asoella can be regareded as a distinct genus, as already assigned in the Treatise.

Pleuromysidia Ichikawa, 1954c, Jour. Inst. Polytech. Osaka City Univ., [G], 1 (1): 52.
Systematic position. —Subjective synonym of Otapiria Marwick, 1935 (Monotidae).
Type-species. —Pleuromysidia dubia Ichikawa, 1954 (original designation).

Somapecten Kimura, 1951, Jour. Fac. Sci. Univ. Tokyo, [2], 7 (7): 347.
Systematic position. —A genus of the Entoliidae.
Type-species. —Somapecten kamimanensis Kimura, 1951 (original designation) [Pl. 3, Figs. 3, 4]
Diagnosis. —Shell equilateral, compressed, subovate, higher than long; auricles subequal, comparatively large, obtusely truncated or rounded; no byssal notch; dorsal margin of left (?) valve elevated above hinge-axis as in Entolium and Pernopecten; surface smooth; auricular crura distinct; a strong bifid tooth in left (?) valve, and a corresponding deep socket and angular ridges on both sides in right (?) valve. (adapted from Tamura, 1959d, p. 61).
Age and distribution. —Upper Jurassic. Japan.
Remarks. —Somapecten is very similar to Entolium Meek, 1865, in every external characters. Though it was referred to the Entoliidae with a query in the Treatise, the familial referrence is undoubted. The strong subumbonal tooth, however, seems to be quite unique in this family.

Explanation of Plate 3 Fig. 1. Oxytoma (Oxytoma) mojsisovicsi Teller. Holotype of O. yeharai (UMUT MM5101), rubber cast from left external mould, ×1. Loc. Umenokidani, Sakawa (Carnian, Kochigatani group). Fig. 2. Entolium inequivalve Hayami. Holotype (UMUT MM3166), rubber cast from right external mould, ×1. Loc. Mitarai, Makito (Callovian, Tetori group). Fig. 3, Somapecten kamimanensis Kimura: type-species of genus Somapecten. Paratype (UMUT MM7120), rubber cast from right(?) internal mould, ×1.5. Loc. Yasukurazawa, Soma (Kimmeridgian, Soma group). Fig. 4. The same species. Holotype (UMUT MM7119), rubber cast from left(?) internal mould, ×1.5. Loc. ditto. Fig. 5. Neithea (Neithea) ficalhoi (Choffat). Two right valves (upper: GK H6276, lower: GK H6636), ×1. Loc. Hiraiga, Tanohata (Aptian, Miyako group). Fig. 6. Neithea (Neithea) nipponica Hayami. Paratype (GK H6632), right valve, ×1. Loc. Aketo, Tanohata (Albian, Miyako group). Fig, 7. Tosapecten suzukii. suzukii (Kobayashi): ncminal subspecies of the type-species of genus Tosapecten. Holotype (UMUT MM5029), rubber cast from right external mould, ×1. Loc. Shimoyama, Sakawa (Carnian, Kochigatani group). Fig. 8. Chlamys mojsisovicsi Kobayashi and Ichikawa. Holotype (UMUT MM5187), rubber cast from left external mould, ×2. Kasayadani, Sakawa (Carnian, Koohigatani group). Fig. 9. Radulonectites japonicus Hayami: type-species of genus Radulonectites. Holotype (UMUT MM2689), rubber cast from left external mould, ×1. Loc. Tsuchizawa, Otari (Pliensbachian?, Kuruma group). Fig. 10. The same species. Paratype (UMUT MM2690), rubber cast from right external mould, ×1. Loc. ditto. Fig. 11. Spondylus (Spondylus) decoratus Nagao. Topotype (GK H6306), left valve, ×1. Loc. Hideshima, Miyako (Aptian, Miyako group). Fig. 12. The same species. Topotype (GK H6309), right valve, ×1. Loc. ditto.

Tosapecten Kobayashi and Ichikawa, 1949a, Japan. Jour. Geol. Geogr., 21 (1-4): 163.
Systematic position. —A genus of the Pectinidae.
Type-species. —Pecten (Velopecten) suzukii Kobayashi, 1931 (original designation) [Pl. 3, Fig. 7]
Diagnosis. —Shell subequilateral exclusive auricles, nearly as long as high; right valve more strongly convex than left; anterior and posterior areas of left valve depressed or slightly concave; anterior auricle larger than posterior; byssal notch moderately deep; surface ornamented with plicated radial costae which are somewhat irregular in breadth and different in arrangement between two valves; scales undeveloped.
Age and distribution. —Carnian-Norian. Japan and east Siberia.
Remarks. —Although Tosapecten was regarded as a subgenus of Weyla Böhm, 1922, in the Treatise, they are morphologically and biogeographically distinct from each other and do not show any direct phyletic relationship. This genus is also well represented in the Upper Triassic of east Siberia.

Neithella Hayami, 1965a. Mem. Fac. Sci. Kyushu Univ., (D), 15 (2): 291.
Systematic position. —A subgenus of Neithea (Pectinidae).
Type-species. —Janira wrightii Shumard, 1860 (original designation).
Diagnosis. —Shell commonly small, comparatively narrow; right valve strongly convex with four to six strongly plicated radial ribs; ribs of left valve correspond with the ribs (instead of grooves) of right valve at the digitated ventral margin; a number of fine radial striae marked on the bottom of interspace and slope of the ribs; left valve nearly flat or slightly concave; auricles highly unequal; anterior auricle pointed forward, much larger than posterior.
Age and distribution. —Neocomian-Turonian. Cosmopolitan.

Radulonectites Hayami, 1957d, Trans. Proc. Pal. Soc, Japan, n. s., (27): 89.
Systematic position. —A genus of the Pectinidae.
Type-species. —Radulonectites japonicus Hayami, 1957 (original designation) [Pl. 3, Figs. 9, 10]
Diagnosis. —Shell inequivalve, inequilateral, higher than long, a little prosocline in young stage but more or less opisthocline in adult; left valve slightly more strongly convex than right; antero-dorsal margin of disk broadly concave as in Pleuronectites and Camtonectes; byssal notch deep, but ctenolium apparently undeveloped; right anterior auricle protruded forward as in Chlamys; surface nearly smooth in young stage but marked with numerous wavy radial grooves after middle stage.
Age and distribution. —Lower Jurassic (Pliensbachian or thereabout). Japan, east Siberia, (?) middle Europe and (?) Argentina.
Remarks. —As originally described and treated in the Treatise, Radulonectites resembles Pleuronectites and Camptonectes but seems to consitute a distinct genus of the Pectinidae. Radulonectites hayamii Polubotko, in Efimova et al., 1968, from the Pliensbachian of east Siberia (Verkhoyansk and Okhotsk areas) is, if not identical with, closely similar to the type-species.

Micronectes Ichikawa and Maeda, 1958b, Jour, Inst. Polytech. Osaka City Univ., [G], 4: 95.
Systematic position. —A genus of the Pectinidae.
Type-species. —Micronectes bellaturus Ichikawa and Maeda, 1958 (original designation) [Pl. 9, Fig. 3]
Diagnosis. —Shell small, slightly prosocline or acline in young stage but a little opisthocline in adult; anterior auricle much larger than posterior; byssal notch distinct; cardinal crura well developed; auricular crura absent; surface smooth except for regular and widely spaced concentric ribs and fine microscopic radiating lines on entire surface.
Age and distribution. —Campanian-Maestrichtian. Japan.
Remarks. —Micronectes was regarded as a subgenus of Eburneopecten Conrad, 1865, in the Treatise, but its generic distinction by original authors seems to be appropriate.

Paranodonta Kobayashi and Suzuki, 1936, Japan. Jour. Geol. Geogr., 13 (3-4): 253.
Systematic position. —A genus of the Unionidae.
Type-species. —Paranodonta otai Kobayashi and Suzuki, 1936 (original designation) [Pl. 9, Fig. 6]
Diagnosis. —Shell subelliptical to subovate, highly convex; umbo subcentral; pseudocardinal teeth rudimentary; posterior teeth absent; ligament subinternal; test very thick;no surface marking except for growth-lines. (according to Kobayashi and Suzuki, 1936, p.253)
Age and distribution. —Lower Cretaceous. Japan.
Remarks. —The above generic diagnosis may be insufficient for its distinction from other genera of the Unionacea, but no diagnostic characters can be added to the original description owing to the poorly preserved material.

Plicatounio Kobayashi and Suzuki, 1936, Japan. Jour. Geol. Geogr., 13 (3-4); 250.
Systematic position. —A genus of the Unionidae.
Type-species. —Plicatounio naktongensis Kobayashi and Suzuki, 1936 (original designation) [Pl. 9, Figs. 4, 5]
Diagnosis. —Shell medium-sized, moderately convex, subelliptical to subtriangular; umbo located submesially or a little anteriorly; pseudocardinal teeth comparatively long, subparallel to pre-umbonal margin, regularly crenulated; posterior teeth strong, may or may not be crenulated; posterior area of surface with several broad radial plications.
Age and distribution. —Lower Cretaceous. South Korea, Japan, north China, Thailand and Sahara.
Remarks. —Plicatounio can be classified into two subgenera: Plicatounio (s. s.) and Kwanmonia.Plicatounio (s. s.) is characterized by elongated and subelliptical outline, large umbonal angle, relatively weak crenulation on pseudocardinal teeth and nearly smooth posterior teeth.

Kwanmonia Ota, 1963, Geol. Rept. Hiroshima Univ., 12: 504.
Systematic position. —A subgenus of Plicatounio (Unionidae).
Type-species. —"Plicatounio" kwanmonensis Ota, 1959 (original designation).
Diagnosis. —Plicatounio with subtriangular outline, relatively small umbonal angle,strong crenulation onpseudocardinal teeth and distinct transverse crenulation on posterior teeth.
Age and distribution. —Lower Cretaceous. Japan.

Nakamaranaia Suzuki, 1943, Jour. Shigenkagaku Kenkyusho, 1 (2): 213.
Systematic position,-A genus of the Unionacea (family uncertain).
Type-species. —Leptesthes chingshanensis Grabau, 1923 (original designation).
Diagnosis. —Shell medium-sized, subtrapezoidal, truncated behind, moderately inflated, posteriorly carinated; test thick; surface smooth; two pseudocardinal teeth in each valve; posterior teeth two in right valve and one in left; umbonal cavity deep. (abridged from Suzuki, 1943, p. 213)
Age and distribution. —Lower Cretaceous. North China, south Korea and Japan.
Remarks. —Nakamuranaia is now an ambiguously defined genus, since Suzuki proposed it on the basis of the material from Korea, whose specific identity with Leptesthes diingshanensis from north China is not necessarily warranted. This taxonomic problem is now under examination in detail by Mr. S. Yang, and the relation and affinity of this genus will be more clarified in the near future.

Nippononaia Suzuki, 1941, Jour. Geol. Soc. Japan, 48 (575): 411.
Systematic position. —A genus of the Trigonioididae.
Type-species. —Unio {Nippononaia) ryosekianus Suzuki, 1941 (original designation) [Pl. 4, Figs. 3, 4]
Diagnosis. —Shell medium-sized, transversely elongated, tapering posteriorly; umbo low, orthogyrous, anteriorly located; surface ornamented with characteristic sculpture scribing Vs in the middle part and upward curving riblets on the antero- and posterodorsal peripheral areas; hinge of unionid type, composed of two crenulated pseudocardinal teeth along pre-umbonal margin and one or two stout non-crenulated posterior teeth along postero-dorsal margin.
Age and distribution. —Lower Cretaceous. Japan, Thailand and (?) Colorado.
Remarks. —Though the type-locality of Nippononaia ryosekiana is still unknown, the rediscovery of firmly identical specimens with the type-species (Hayami and Ichikawa, 1965) made the generic characters more clear. In the Treatise, Nippononaia was still placed in the Unionidae as done by some previous Japanese authors, but it is certainly referable to the Trigonioididae.

Explanation of Plate 4 Fig. 1. Antiquilima nagatoensis Hayami. Holotype (UMUT MM3421), rubber cast from left external mould, ×1. Loc. Higashinagano, Toyoda (Sinemurian, Toyora group). Fig. 2. Amphidonte (Amphidorite} subhaliotoidea (Nagao). Left valve (GK H6343), ×1. Loc. Hiraiga, Tanohata (Aptian, Miyako group). Figs. 3, 4. Nippononaia ryosekiana (Suzuki): type-species of genus Nippononaia. Holotype (UMUT MM7000), rubber cast of left internal mould (fig. 3) and rubber cast of left external mould of the same individual (fig. 4), ×1. Loc. unknown (Katsuuragawa area or Sanchu area). Fig. 5. Trigonioides (Trigonioides) kodairai Kobayashi and Suzuki: type-species of genus Trigonioides. Topotype (Kyungpook National Univ., Daegu, Korea, KPE 1001), right valve, ×1.5. Loc. Sumoondong, Keisyo-nan-do (Lower Cretaceous, Naktong group). Fig. 6. The same species. Topotype (KPE 1007), rubber cast from right internal mould, ×1.5. Loc. ditto. Fig. 7. Trigonioides (Kumamotoa) mifunensis Tamura: type-species of Kumamotoa. Topotype (KE not registered), rubber cast from right internal mould, ×1. Loc. Tashiro, Kosa (Cenomanian-Turonian, Mifune group). Fig. 8. The same species. Topotype (KE not registered), rubber cast from left internal mould, xl. Loc. ditto. Fig. 9. Cardimoides varidus Hayami. Rubber cast from bivalved internal mould (UMUT MM2641), ×1. Loc. Tsuchizawa, Otari (Pliensbachian?, Kuruma group) Fig. 10. Palaeopharus maizurensis Kobayashi and Ichikawa. Holotype (UMUT MM5301), rubber cast of bivalved internal mould, ×1.5. Loc. Maizuru (Carnian, Nabae group).

Trigonioides Kobayashi and Suzuki, 1936, Japan. Jour. Geol. Geogr., 13 (3-4): 249.
Systematic position. —A genus of the Trigonioididae.
Type-species. —Trigonioides kodairai Kobayashi and Suzuki, 1936 (original designation) [Pl. 4, Figs. 5, 6]
Diagnosis. —Shell medium- or large-sized, subelliptical to trigonally ovate; umbo prominent, orthogyrous, located submesially or a little anteriorly; median part of disk ornamented with strong ribs which converge ventrally to form acute Vs; antero- and postero-dorsal peripheral areas having upward curving riblets which form reversed Vs with the ribs on median part; hinge plate thickened along preumbonal margin; pseudo- cardinal teeth two to five in right valve and three or four in left, radiating from umbo, strongly crenulated; posterior teeth short, also transversely crenulated; lunule and escutcheon may or may not be demarcated.
Age and distribution. —Lower Cretaceous-Upper Cretaceous. South Korea, Japan, north China (Manchuria), south China (Yunnan), Laos and Thailand.
Remarks. —The type-species of Trigonioides was recently redescribed by Yang (1974) on the basis of many well-preserved topotype specimens, and the generic characters became much clear. Various disputed opinions have been presented as to the classification and phylogeny of Trigonioides and allied genera (Cox, 1955; Kobayashi, 1956,1963, 1968; Ota, 1959b, 1963; Hayami and Ichikawa, 1965; Martinsson, 1965; Tamura, 1970; etc.). Although the ontogenetic transformation of hinge teeth should be further examined, the radiating pseudocardinal teeth of Trigonioides (s. s.) was, I presume, originated from the subparallel teeth along the pre-umbonal margin as seen in Plicatoumo and Nippononaia, which seem to be more closely related to unionid teeth rather than to trigoniid teeth. It is now maintained that the similarity of surface ornamentation between Trigonioides and Nippononaia is not superficial. Trigonioides seems to be classifiable into four subgenera: Hoffetrigonia Suzuki, 1940, Wakinoa Ota, 1963, Kumamotoa Yang, 1974, and Trigonioides (s. s.). Trigonioides (s. s.) is characterized by the medium size, suboval outline, moderately developed radiating pseudocardinal teeth (three in right valve and two or three in left) and crenulated posterior teeth (two in right and one in left).

Wakinoa Ota, 1963, Geol. Rept. Hiroshima Univ., 12: 504.
Systematic position. —A subgenus of Trigonioides (Trigonioididae).
Type-species."Nippononaia" wakinoensis Ota, 1959 (original designation).
Diagnosis. —Shell medium-sized, transversely elongated; two pseudocardinal tee th subparallel to pre-umbonal margin, narrow, finely crenulated; subumbonal teeth, if present, much smaller than in Trigonioides (s. s.); posterior teeth two in left valve and one or two in right, long, finely crenulated; surface ornament as in Trigonioides (s. s.).
Age and distribution. —Lower Cretaceous. Japan.
Remarks. —This subgenus appears to be ancestral to Trigonioides (s. s.) and morphologically intermediate between Nippononaia and Trigonioides (s. s.). Some recent authors (Tamura, 1970; Yang, 1974) regarded Wakinoa as a distinct genus of the Trigonioididae.

Kumamotoa Yang, 1974, Trans. Proc. Pal. Soc. Japan, n, s., (95): 405.
Systematic position. —A subgenus of Trigonioides (Trigonioididae).
Type-species. —Trigonioides mifunensis Tamura, 1970 (original designation) [Pl. 4, Figs, 7,8]
Diagnosis. —Shell relatively large, subtrapezoidal or subtrigonal; surface ornament similar to that of Trigonioides (s. s.); hinge plate much thickened along umbonal margin, provided with three or four strong pseudocardinal teeth which radiate from umbo and are strongly crenulated; posterior teeth comparatively short.
Age and distribution. —Lower Cretaceous-Upper Cretaceous. Japan.
Remarks. —Kumamotoa shows the best developed subumbonal teeth among various groups of Trigonioides, and was possibly derived from Trigonioides (s. s.).

Cardinioides Kobayashi and Ichikawa, 1952a, Japan. Jour. Geol. Geogr., 22: 65.
Systematic position. —A genus of the Pachycardiidae.
Type-species. —Cardinioides japonicus Kobayashi and Ichikawa, 1952 (original designation).
Diagnosis. —Shell medium-sized, trigonally ovate to ovate, moderately inflated; no lunule; no escutcheon; ligament external; left valve with large trigonal median tooth commonly carved by several subvertical grooves; right valve with two teeth on both sides of the corresponding socket; posterior lateral tooth distinct in right valve, far isolated from cardinals, rounded, placed near posterior adductor scar; surface smooth except for growth-lines.
Age and distribution. —Carnian-middle Lower Jurassic. Japan and (?) Thailand.
Remarks. —This genus was referred to the Cardiniidae by Kobayashi and Ichikawa (1952a) and Hayami (1957c), but is more appropriately referred to the Pachycardiidae, as suggested by Hayami (1961c, p. 258) and in the Treatise. Because the materials of the type-species are poorly preserved, the above diagnostic characters are partly supplemented by a related Lower Jurassic species, Cardinioides varidus Hayami, 1957 [Pl. 4, Fig. 9]

Okunominetania Ichikawa, 1954d, Jour. Inst, Polytech. Osaka City Univ., [G], 2; 62.
Systematic position. —A subgenus of Neosckizodus (Myophoriidae).
Type-species. —Myophoria okunominetaniensis Ichikawa, 1949 (original designation).
Diagnosis. —Neoschizodus with shallow transverse lateral crenulation on cardinal teeth. (according to Ichikawa, 1954d, p. 62)
Age and distribution. —Triassic. Japan.
Remarks. —Okunominetania was regarded in the Treatise as a synonym of Neoschizodus.In fact, the distinction of this subgenus from Neoschizodus (s. s.) is still obscure, since the hinge structure, particularly presence or absence of transverse crenulation on the cardinal teeth, has not been sufficiently examined in most species of Neoschizodus.

Kumatrigonia Tamura, 1959a, Mem, Fac. Educ. Kumamoto Univ., 7: 213.
Systematic position. —A subgenus of Frenguelliella (Trigoniidae).
Type-species. —Frenguelliella (Kumatrigonia) tanourensis Tamura, 1959 (original designation) [Pl. 5, Fig. 7]
Diagnosis. —Shell trigonal, moderately convex, with orthogyrous umbo, strong marginal carina, fine but distinct escutcheon carina, lanceolate escutcheon costellated by fine striae, transversely costellated area and plain regular concentric costae on disk which are connected with the costellae on area. (according to Tamura, 1959a, p. 213)
Age and distribution. —Carnian. Japan.
Remarks. —Kumatrigonia was regarded in the Treatise as a subgenus of Trigonia but is here treated as a subgenus of Frenguelliella which is taken as a distinct genus.

Explanation of Plate 5 Fig. 1. Trigonia sumiyagura Kobayashi and Kaseno. Holotype (UMUT MM430) × rubber cast from left external mould, ×1 Loc. Kosaba, Karakuwa (Bajocian, Karakuwa group). Fig. 2. The same species. Rubber cast from left external mould (UMUT MM2962), ×1. Loc. Akaiwazaki, Shizukawa (Bajocian, Hashiura group). Fig. 3. Geratrigonia hosourensis (Yokoyama): type-species of genus Geratrigonia. Lectoype (UMUT MM7174), left valve of a bivalved specimen, ×1. Loc. Niranohama, Shizukawa (Hettangian, Shizukawa group). Fig. 4. Latitrigonia pyramidalis Kobayashi and Tamura: type-species of genus Latitrigonia. Holotype (UMUT MM4378), rubber cast from left external mould, ×1.5. Loc. Nodezawa, Soma (Bajocian, Soma group). Fig. 5. Ibotrigonia masatanii Kobayashi and Tamura: type-species of genus Ibotrigonia. Rubber cast from left external mould (UMUT MM4383), ×1.5. Loc. Sugaya, Soma (Bathonian ?, Soma group). Fig, 6. Minetrigonia hegiensis hegiensis (Saeki): nominal subspecies of the type-species of genus Minetrigonia. Holotype (UMUT MM5026), rubber cast from left external mould, ×1.5. Loc. Nukata, Yakuno (Carnian, Heki formation). Fig. 7. Frenguelliella (Kumatrigonia) tanonrensis Tamura: type-species of subgenus Kumatrigonia. Holotype (UMUT MM3080), right valve, ×1. Loc. Okiba, Tanoura (Carnian, Kochigatani group). Fig. 8. Vaugonia (Hijitrigonia) geniculata Kobayashi and Mori: type-species of subgenus Hijitrigonia. Paratype (UMUT MM4338), rubber cast from left external mould, ×1. Loc. Akaiwazaki, Shizukawa (Bajocian, Shizukawa group). Fig, 9. Myophorella (Promyophorella) sigmoidalis Kobayashi and Tamura: type-species of subgenus Promyophorella. Holotype (UMUT MM4350), rubber cast from right external mould, ×1. Loc. Akaiwazaki, Shizukawa (Bajocian, Hashiura group). Fig. 10. Nipponitrigonia kikuchiana (Yokoyama); type-species of genus Nipponitrigonia. Right valve (GK not registered), ×1. Loc. Koikorobe, Tanohata (Aptian, Miyako group). Fig. 11. Steinmanella (Yeharlla) ainuana (Yabe and Nagao). Left valve of a bivalved specimen (GK H6054), ×1. Loc. Katsurazawa-dam, Ikushumbetsu (Cenomanian, Middle Yezo group). Fig. 12. Steinmanella (Yeharelld) japonica (Yehara): type-species of subgenus Yeharella. Rubber cast from left external mould (UMUT MM4413), ×1. Loc. Aonami, Onsen (Campanian, Izumi group). Fig. 13. Steinmanella (Setotrigonid) shinoharai Kobayashi and Amano: type-species of subgenus Setotrigonia. Holotype (UMUT MM4424), rubber cast from right external mould, ×1. Loc. Tsubasayama, Hiketa (Campanian, Izumi group).

Geratrigonia Kobayashi in Kobayashi and Mori, 1954, Japan. Jour, Geol. Geogr., 25 (3-4): 171.
Systematic position. —A genus of the Trigoniidae.
Type-species. —Trigonia hosourensis Yokoyama, 1904 (original designation) [Pl. 5, Fig. 3]
Diagnosis. —Shell medium-sized, ovately trigonal, with orthogyrous or slightly opisthogyrous umbo, obtuse marginal carina, smooth and bipartite area, lanceolate and smooth escutcheon and somewhat irregular subconcentric ribs on disk. (abridged from Kobayashi and Mori, 1954, p. 171)
Age and distribution. —Hettangian-Toarcian. Japan.
Remarks. —Geratrigonia is quite unique not only in the morphology but also in the mode of occurrence. Unlike other trigoniid genera it is found primarily in cyrenoid-beds which indicate embayment condition and somewhat decreased or unstable salinity.

Latitrigonia Kobayashi in Kobayashi and Tamura, 1957, Japan. Jour. Geol. Geogr., 28 (1-3); 36.
Systematic position. —A genus of the Trigoniidae.
Type-species. —Latitrigonia pyramidalis Kobayashi and Tamura, 1957 (original designation) [Pl. 5, Fig. 4]
Diagnosis. —Shell small-sized, subquadrate or suborbicular, with diagonal marginal carina; area often very large, nearly smooth; escutcheon narrow, smooth; costae on disk parallel to ventral margin, each sometimes thickened into a node at the posterior end. (according to Kobayashi and Tamura, 1957, p. 36)
Age and distribution. —Bajocian-Oxfordian. Japan.

Ibotrigonia Kobayashi in Kobayashi and Tamura, 1957, Japan, Jour. Geol. Geogr., 28 (1-3): 38.
Systematic position. —A genus of the Trigoniidae.
Type-species. —Ibotrigonia masatanii Kobayashi and Tamura, 1957 (original designation) [Pl. 5, Fig. 5]
Diagnosis. —Shell small-sized, roundly trigonal; umbo submedian, opisthogyrous; escutcheon narrow, smooth; area smooth or transversely costellate; three carinae tuberculate; costae on disk subconcentric, small in number, broken into tubercles on posterior side. (according to Kobayashi and Tamura, 1957, p 38)
Age and distribution. —Bathonian (or thereabout). Japan.

Nipponitrigonia Cox, 1952, Proc. Malac. Soc. London, 29, (2-3): 52.
Systematic position. —A genus of the Trigoniidae.
Type-species. —Trigonia kikuchiana Yokoyama, 1891 (original designation) [Pl. 5, Fig. 10]
Diagnosis. —Shell trigonally ovate, short, moderately inequilateral; umbo prominent; marginal carina obtuse, rounded off in later growth-stage; area smooth, escutcheon not defined; disk with concentric costae near umbo, but later smooth or only with impersistent concentric costae on its anterior part. (adapted from Cox, 1952, p. 52)
Age and distribution. —Bathonian-Cenomanian. Japan and Philippines.

Minetrigonia Kobayashi and Katayama, 1938, Proc. Imp. Acad. Tokyo, 14: 187.
Systematic position. —A genus of the Trigoniidae.
Type-species. —Trigonia hegiensis Saeki, 1925 (original designation) [Pl. 5, Fig. 6]
Diagnosis. —Shell roundly subtrigonal to subtrapezoidal; umbo nearly orthogyrous; posterior area well defined, forming an obtuse angle with disk, provided with a median furrow, ornamented with fine and densely set striae in form of oblique lattice; disk covered with plain regular concentric costae and radial ribs. (abridged from Kobayashi and Ichikawa, 1952a, p. 69)
Age and distribution. —Carnian-Norian. Japan, Bear Island, east Siberia, British Columbia, Peru and New Zealand.
Remarks. —Minetrigonia was originally established as a subgenus of Trigonia, but most recent investigators share the opinion that it constitutes a very characteristic genus and one of the earliest trunk in the evolution of the Trigoniidae.

Hijitrigonia Kobayashi in Kobayashi and Mori, Japan. Jour, Geol. Geogr., 26 (1-2): 84.
Systematic position. —A genus of Vaugonia (Trigoniidae).
Type-species. —Vaugonia (Hijitrigonia) geniculata Kobayashi and Mori, 1955 (original designation) [Pl. 5, Fig. 8]
Diagnosis. —Vaugonia with geniculate anterior branches of V-costae which are often finer and more numerous than posterior ones. (according to Kobayashi and Mori, 1955, p. 84).
Age and distribution. —Hettangian-Bajocian. Japan, (?) western Europe and (?) Argentina.
Remarks. —Although some authors (e.g., Treatise) did not accept the subgeneric distinction, the name of Hijitrigonia seems to be applicable for a specialized species group of Vaugonia with geniculate anterior ribs.

Promyophorella Kobayashi and Tamura, 1955, Japan. Jour. Geol. Geogr., 26 (1-2): 96.
Systematic position. —A subgenus of Myophorella (Trigoniidae).
Type-species. —Myophorella (Promyophorella) sigmoidalis Kobayashi and Tamura, 1955 (original designation) [Pl. 5, Fig. 9]
Diagnosis. —Myophorella, of relatively small-size, with narrow costae on disk, which have numerous small tubercles regularly arranged on the top, though the tubercles are obsolete in some later species, (emended from Kobayashi and Tamura, 1955, p. 96)
Age and distribution. —Toarcian-Lower Neocomian. Japan, western Europe, Argentina, western Canada, Philippines and east Siberia.
Remarks. —Promyophorella was regarded by some authors (e.g., Treatise) as synonymous with Myophorella. I agree, however, with most Japanese authors in considering that Myophorella (s. s.), Promyophorella and Haidaia are subgenerically separable from one another, because of the differences in the surface ornamentation and geographic distribution.

Yeharella Kobayashi and Amano, 1955, Japan. Jour. Geol. Geogr., 26 (3-4): 200.
Systematic position. —A subgenus of Steinmanella (Trigoniidae).
Type-species. —Trigonia japonica Yehara, 1923 (original designation) [Pl. 5, Fig. 12]
Diagnosis. —Shell large-sized, resembling Steinmanella (s. s.) but different in the obsolete carinae and transverse costellae on area; oblique costae on disk frequently broken into large and somewhat irregularly spaced tubercles. (adapted from Kobayashi and Amano, 1955, p. 200)
Age and distribution. —Cenomanian-Maestrichtian. Japan and Saghalin.

Setotrigonia Kobayashi and Amano, 1955, Japan. Jour. Geol. Geogr., 26 (3-4): 206.
Systematic position. —A subgenus of Steinmanella (Trigoniidae).
Type-species. —Steinmanella (Setotrigonia) shinoharai Kobayashi and Amano, 1955 (original designation) [Pl. 5, Fig. 13]
Diagnosis. —Shell large-sized, resembling Steinmanella (s. s.) and Yeharella, but different in the complete effacement of carinae and the transverse ribs on area which are connected with costae on disk at the obtuse angulation in place of marginal carina; costae and costellae broken into cords at different intervals in grown stage. (emended from Kobayashi and Amano, 1955, p. 206)
Age and distribution. —Campanian. Japan.

Microtrigonia Nakano, 1957a, Japan. Jour. Geol. Geogr., 28 (1-3): 116.
Systematic position. —A genus of the Trigoniidae.
Type-species. —Microtrigonia amanoi Nakano, 1957 (original designation) [Pl. 9, Fig. 8]
Diagnosis. —Shell very small, suborbicular, moderately convex; umbo broad, imprminent, slightly opisthogyrous, with several plain concentric costae; carinae obscure except near umbo; area wide, with often tuberculated transverse costellae; escutcheon narrow, with transverse costellae; costae on disk composed of two series: anterior series oblique and tuberculate; posterior series also tuberculate, radial or subvertical. (abridged from Nakano, 1957a, p. 116)
Age and distribution. —Campanian-Maestrichtian. Japan.

Crenocolus Ichikawa, in Ichikawa and Maeda, 1966, Prof. Matsushita Mem. Vol., Kyoto: 235.
Systematic position. —A subgenus of Clisocolus (Mactromyidae).
Type-species. —Clisocolus (Crenocolus) crenulatus Ichikawa and Maeda, 1966 (original designation) [Pl. 10, Figs. 3a-c]
Diagnosis. —Clisocolus with crenulated inner margin of the valve. It becomes comparatively large. (according to Ichikawa and Maeda, 1966, p. 235)
Age and distribution. —Campanian-Maestrichtian. Japan.

Explanation of Plate 10 Fig. 1. Izumicardia parva Ichikawa and Maeda: type-species of genus Izumicardia. Holotype (OCU MM286), rubber cast from right external mould, × 2. Loc. Mikumayama, Sumoto (Campanian, Izumi group). Reproduced from Ichikawa and Maeda, 1963, pl. 9, fig. Id. Fig. 2. The same species. Topotype (OCU MM291), rubber cast from right internal mould, × 2. Loc. ditto. Reproduced from Ichikawa and Maeda, 1963, pl. 9, fig. 7a. Fig. 3. Clisocolus (Crenocolus) crenulatus Ichikawa and Maeda: type-species of subgenus Crenocolus. Holotype (OMM F1025), right valve, ×1. Loc. Azenotani, Izumi mountains (Campanian, Izumi group). Reproduced from Ichikawa and Maeda, 1966, pl. 7, figs. 1a-c. 3a: external view, 3b: internal view, 3c: upper view. Fig. 4. Izumia trapezoidalis Ichikawa and Maeda; type-species of genus Izumia, Topotype (OCU MM276), clay cast from left internal mould, × 3. Loc. Mikumayama, Sumoto (Campanian, Izumi group). Reproduced from Ichikawa and Maeda, 1963, pl. 10, fig. 8b. Fig. 5. The same species. Holotype (OCU MM262), right internal mould, × 2. Loc. ditto. Reproduced from Ichikawa and Maeda, 1963, pl. 10, fig. 1a. × Fig. 6. The same species. Topotype (OCU MM337), clay cast from right internal mould, × 3. Loc. ditto. Reproduced from Ichikawa and Maeda, 1963, pl. 10, fig. 7b. Fig. 7. Eoursivivas matsumotoi (Hase): type-species of genus Eoursivivas. Right internal mould (GF K63050), × 2. Loc. Sakayori-agaru, Yatsushiro (Neocomian, Kawaguchi formation). Reproduced from Ota, 1964, pl. 21, fig. 1. Fig. 8. The same species. Left internal mould (GF K63061), ×1.8. Loc. ditto. Reproduced from Ota, 1964, pl. 21, fig. 3. Fig. 9. Pulsidis nagatoensis Ota: type-species of genus Pulsidis, Holotype (GF Y6301), right internal mould, × 2.5. Loc. Yoshimo, Shimonoseki (Neocomian, Yoshimo formation). Fig. 10. Nipponicorbula mifunensis Ota: type-species of genus Nipponicorbula. Paratype (GF M63002), right valve, × 3. Loc. Asanoyabu, Kamimasuki (Cenomanian or Turonian, Mifune group). Reproduced from Ota, 1964, pl. 21, fig. 19. Fig. 11. Praecaprotina yaegashii (Yehara): type-species of genus Praecaprotina. Syntype (IGPS not registered), bivalved specimen, ×1. Loc. Moshi, Iwaizumi (Aptian, Miyako group). Reproduced from Yabe and Nagao, 1926, pl. 7, fig. 1.

Izumicavd.ia Ichikawa in Ichikawa and Maeda, 1963, Jour. Geosci. Osaka City Univ., 7(5): 118.
Systematic position. —A genus of the Carditidae.
Type-species. —Izumicardia parva Ichikawa and Maeda, 1963 (original designation) [Pl. 10, Figs. 1, 2]
Diagnosis. —Shell small-sized, moderately convex, roundly subquadrate, subequilateral, with prominent and prosogyrous umbo; lunule distinct, deeply excavated; escutcheon present; hinge of astartoid type, formulated: AI 3a 3b 5b PIII/AII 2 4b (PII); inner ventral margin denticulate. (abridged from Ichikawa and Maeda, 1963, p. 118)
Age and distribution. —Campanian-Maestrichtian. Japan.

Yabea Hayami, 1965b, Mem. Fac. Sci. Kyushu Univ., [D], 17 (2): 92.
Systematic position. —A subgenus of Astarte (Astartidae).
Type-species. —Astarte shinanoensis Yabe and Nagao, 1926 (original designation) [Pl. 6, Figs. 5a, b]
Diagnosis. —Medium-sized Astarte with highly inequilateral outline, strong convexity, subterminal and extremely prosogyrous umbo, deeply excavated pre-umbonal margin and fine ventral crenulation; hinge of astartoid type, formualted: AI (3a) 3b 5b (PIII)/ AII 2 4b PII; cardinal teeth 2, 3b and 4b very strong, highly elevated; umbonal cavity profound. (abridged from Hayami, 1965b, p. 92)
Age and distribution. —Oxfordian-Albian. Japan, Philippines and Middle Europe.

Explanation of Plate 6 Fig. 1. Pterotrigonia (Pterotrigonia) hokkaidoana (Yehara). Left valve (GK not registered), × . Loc. Haipe, Tanohata (Aptian, Miyako group). Fig. 2. Sphaeriola nipponica Hayami. Holotype (UMUT MM3450), rubber cast from right internal mould, ×1. Loc. Higashinagano, Toyoda (Sinemurian, Toyora group). Fig. 3. Astarte (Astarte) subsenecta Yabe and Nagao. Rubber cast from left external mould (GK H6405), ×1.5. Loc. Ichinose-bashi, Nakazato (up. Neocomian or Aptian, Ishido formation). Fig. 4. The same species. Rubber cast from left internal mould (GK H6403), ×1.5. Loc. ditto. Fig. 5. Astarte (Yabed) shinanoensis Yabe and Nagao: type-species of subgenus Yabea. Rubber cast from internal (5a) and external (5b) moulds of a left valve (GK H6652), ×1.5. Loc. ditto. Fig. 6. Eriphyla (Eriphyla) miyakoensis Nagao. Topotype (GK H6433), right valve, ×1. Loc. Hiraiga, Tanohata (Aptian, Miyako group). Fig. 7. The same species. Topotype (GK H6436), right valve, ×1.5. Loc. ditto. Fig. 8. Anthonya subcantiana Nagao. Topotype (GK H6464), right valve, ×1.5. Loc, Haipe, Tanohata (Aptian, Miyako group). Fig. 9. The same species. Left valve (GK H6471), ×1.5. Loc. Hiraiga, Tanohata (Aptian, Miyako group). Fig. 10. Cardinia. toriyamai Hayami. Holotype (UMUT MM2918), plaster cast from left internal mould, ×1. Loc. Higashinagano, Toyoda (Sinemurian, Toyora group). Fig. 11. The same species. Paratype (UMUT MM2917), plaster cast from right external mould, ×1. Loc. ditto. Fig. 12. Minepharus triadicus (Tokuyama): type-species of genus Minepharus. Holotype (UMUT MM4470), left valve, ×1. Loc. Hirabarazaka, Mine (Carnian, Mine group). Fig. 13. The same species. Paratype (UMUT MM4475), rubber cast from left internal mould, ×1. Loc. ditto. Fig. 14. Globocardium sphaeroideum (Forbes): type-species of genus Globocardium. Left valve (GK not registered), ×1. Loc. Hiraiga, Tanohata (Aptian, Miyako group).

Miyakoella Hayami, 1965b, Mem. Fac. Sci. Kyushu Univ., (D), 17 (2): 100.
Systematic position. —Subjective synonym of Eriphyla (Astartidae).
Type-species. —Astarte miyakoensis Nagao, 1934 (original designation).
Remarks. —Originally Miyakoella was proposed as a subgenus of Eriphyla, but was later regarded in the Treatise as a synonym of Eriphyla (s. s.). The development of inner marginal crenulations, which was once regarded as diagnostic in Miyakoella, should be taxonomically evaluated in the future, because this character seems to be often variable within one astartid species.

Minepharus Tokuyama, 1958, Trans. Proc. Pal. Soc. Japan, n. s., (32): 296.
Systematic position. —A genus of the Cardiniidae.
Type-species. —Palaeopharus (Minepharus) triadicus Tokuyama, 1958 (original designation).
Diagnosis. —Shell medium-sized, oblong; umbo anteriorly located, prosogyrous; ligament, escutcheon and pseudo-lunule of Cardinia-type; anterior adductor scar strongly impressed, smaller than posterior, accompanied by a strongly impressed pedal retractor scar; one cardinal tooth (3b) in right valve and two (2 and 4b) in left; lateral teeth also of Cardinia-type; ornament consisting of several broad radial costae on posterior area and irregular growth lamellae on entire surface.
Age and distribution. —Carnian. Japan.
Remarks. —Mineplarus was originally proposed as a subgenus of Palaeopharus Kittl, 1907. As originally suggested, however, it shares many common essential characters with Cardinia and is certainly referable to the Cardiniidae. On the other hand, Palaeo pharus belongs to the Actinodontophoridae and is not directly related to Minepharus as recognized from the difference of hinge structure. One of the paratype specimens of the type-species (Tokuyanma, 1958, pl. 43, fig. 5) is immature and shows "pseudoradial ribs", a characteristic feature of Palaeopharus. However, it is probably referable to Palaeopharus oblongatus, which occurs with the present type-species at the same locality.

Globocardium Hayami, 1965b, Mem. Fac. Sci. Kyushu Univ., [D], 17 (2): 116.
Systematic position. —A genus of the Cardiidae.
Type-species. —Cardium sphaeroideum Forbes, 1845 (original designation) [Pl. 6, Fig. 14]
Diagnosis. —Shell large, globose, more or less higher than long; disk ornamented with widely spaced concentric costae; posterior area nearly smooth, without any conspicuous radial ribs; posterior carina obsolete, but there are three narrow internal ridges, two in right valve and one in left, along the boundary between disk and posterior area; inner ventral margin smooth; hinge similar to that of Protocardia.
Age and distribution. —Upper Neocomian-Albian. West Europe, Crimea, Caucasus, Japan and Tanzania.
Remarks. —Globocardium was originally proposed as a subgenus of Protocardia. Recently Palmer (1974) has clarified its ancestry and regarded it as a distinct genus of the Proto cardiinae. His scheme of classification is here accepted.

Yokoyamaina Hayami, 1958b, Japan. Jour. Geol. Geogr., 29 (1-3): 23.
Systematic position. —A subgenus of Integricardium (Cardiidae).
Type-species. —Cyrena elliptica Yokoyama, 1904, non Dunker, 1843 (original designa tion) (= Yokoyamaina hayamii Keen and Casey in Cox et al., 1969) [Pl. 7, Figs. 1, 2]
Diagnosis. —A subgenus of Integricardium with subelliptical or suborbicular outline, distinct pallial sinus and elongated parvincular ligament attached to a shallow groove on nymph; hinge of cyclodont-type, with well developed conical cardinal teeth 2 and 3b, ill-defined 3a and 4b and stout tubercular lateral teeth AI and PI; surface with weak radial threads on posterior or over entire surface. (adapted from Hayami, 1972, p. 205)
Age and distribution. —Hettangian-Toarcian. Japan and south Viet-Nam.
Remarks. —Yokoyamaina was originally proposed for a strange sinupalliate heterodont species, Cyrena elliptica Yokoyama, 1904, from the lower Lias of north Japan. Then, it was referred, with some doubt, to the Arcticidae. In the Treatise, Keen and Casey treated Yokoyamaina as a member of the Corbiculidae also with a query and gave a new name to the type-species, because Yokoyama's name was preoccupied by an in dependent cyrenoid species of Dunker (1843). On the occasion of describing a related species from south Viet-Nam (Hayami, 1972), I reexamined the type-species and reached the conclusion that Yokoyamaina is actually a cardiid and more appropriately regarded as a subgenus of Integricardium.

Explanation of Plate 7 Fig. 1. Integricardium (Yokoyamaina) hayamii (Keen and Casey): type-species of subgenus Yokoyamaina. Topotype (UMUT MM2864), left valve, ×1. Loc. Niranohama, Shizukawa (Hettangian, Shizukawa group). Fig. 2. The same species. Topotype (UMUT MM2867), rubber cast from right internal mould, ,×1. Loc. ditto. Fig. 3. Somarctica abukumensis (Tamura): type-species of genus Somarctica. Topotype (UMUT MM3320), rubber cast from left internal mould, ×1. Loc. Yamashita, Soma (Kimmeridgian, Soma group). Fig. 4. Sakawanella triadica Ichikawa; type-species of genus Sakawanella. Holotype (UMUT MM5169), rubber cast from left internal mould, × 2. Loc. Okunominetani, Sakawa (Carnian, Kochigatani group). Fig. 5, Ptychomya densicostata Nagao. Right valve of a bivalved specimen (GK H6513), Kinchakuiwa, Ofunato (upper Neocomian, Ofunato group). Fig. 6. Eomiodon lunulatus (Yokoyama). Topotype (UMUT MM2840), right valve, ×1.5. Loc. Niranohama, Shizukawa (Hettangian, Shizukawa group). Fig. 7. The same species. Topotype (UMUT MM2843), rubber cast from bivalved internal mould, × 1.5. Loc. ditto. Fig. 8. Protocyprina naumanni (Neumayr). Topotype (GK H6739), rubber cast from left internal mould, ×1. Loc. Yanagidani, Katsuuragawa (Neocomian, Tatsukawa formation). Fig. 9. The same species. Topotype (GK H6726), left valve of a bivalved specimen, ×1. Loc. ditto. Fig. 10. Costocyrena matsumotoi Hayami: type-species of genus Costocyrena. Holotype (GK H6502), rubber cast from right external mould, ×1.5. Loc. Miyaji, Yatsushiro (Albian, Yatsushiro formation). Fig. 11. The same species. Paratype (GK H6503), rubber cast from right internal mould, ×1.5. Loc. ditto. Fig. 12. Pseudasaphis japonica Matsumoto: type-species of genus Pseudasaphis. Syntype (UMUT MM7749), rubber cast from right external mould, ×1.5. Loc. Narukogawa, Gosyonoura (Cenomanian, Gosyonoura group). Fig. 13, Crenotrapezium kurumense kurumense Hayami: nominal subspecies of the type species of genus Crenotrapezium. Holotype (UMUT MM2823), left valve, ×1.5. Loc. Tsuchizawa, Otari (Pliensbachian ?, Kuruma group). Fig. 14. The same species. Topotype (UMUT MM2829), rubber cast from right internal mould, ×1.5. Loc. ditto.

Sakawanella Ichikawa, 1950, Jour. Fac. Sci. Univ. Tokyo, [2], 7 (3): 245.
Systematic position. —A genus of the Tancrediidae.
Type-species. —Sakawanella triadica Ichikawa, 1950 (original designation) [Pl. 7, Fig. 4]
Diagnosis. —Shell small, elongate sub quadrate; umbo orthogyrous, submesially placed; right valve with two cardinal and one posterior lateral teeth; left valve with one cardinal; no anterior lateral teeth; adductor scars and pallial line strongly impressed, as in Tancredia; two internal grooves diverging from beak.
Age and distribution. —Carnian. Japan.

Somarctica Tamura, 1960c, Trans. Proc. Pal. Soc. Japan, n. s., (39): 288.
Systematic position. —A genus of the Arcticidae.
Type-species. —Arctica (Somarctica) abukumensis Tamura, 1960 (original designation) [Pl. 7, Fig. 3]
Diagnosis. —Shell large, subovate, moderately convex; umbo strongly prosogyrous; dentition of cyprinoid-type, as formulated: 3a 1 3b PI/AII-2a 2b 4b; cardinal 1 tuber culiform, not fully developed, 3a distinct, 3b trigonal, wide and bifid, PI short, 2a not clearly differentiated from AII, 4b obliquely elongated.
Age and distribution. —Kimmeridgian. Japan.
Remarks. —Tamura (1960c) proposed Somarctica as a subgenus of Arctica, but, as re garded in the Treatise, it is here treated as a distinct genus.

Izumia Ichikawa in Ichikawa and Maeda, 1963, Jour. Geosci. Osaka City Univ., 7 (5): 122.
Systematic position. —A genus of the Arcticidae.
Type-species. —Izsumia trapezoidalis Ichikawa and Maeda, 1963 (original designation) [Pl. 10, Figs. 4-6]
Diagnosis. —Shell trapeziform, moderately convex, posteriorly carinated; lunule wide but bluntly limited; no escutcheon; surface nearly smooth, but numerous close-set fine radial striae appearing on weathered surface; hinge of cyrenoid type, as formulated: AIII AI 3a 1 3b PI (PIII)/AII 2a 2b 4b PII; anterior lateral teeth tuberculiform, not clearly differentiated from anterior cardinals; posterior lateral teeth not much de generated; inner ventral margin finely crenulated.
Age and distribution. —Campanian. Japan.
Remarks. —While Izumia was originally proposed as a genus of the Veneridae, in the Treatise it was placed in the Arcticidae with a query. In fact, the discrimination of the two families is often difficult as to Cretaceous material, since the Veneridae are believed to have been derived polyphyletically from the Arcticidae in this period. The mode of pallial line, which is one of the criteria for the discrimination, is unknown in the type material. In the present study Izumia is tentatively included in the Arctici dae, because such a posteriorly carinated shell and persistent posterior lateral teeth are more commonly met with in this family.

Costocyrena Hayami, 1965b, Mem. Fac. Sci. Kyushu Univ., [D], 17 (2): 130.
Systematic position. —A genus of the Neomiodontidae.
Type-species. —Costocyrena matsumotoi Hayami, 1965 (original designation) [Pl. 7, Figs. 10, 11]
Diagnosis. —Shell medium-sized, trigonally ovate or subcuneiform, weakly carinated; umbo prosogyrous; lunule large, shallow, nearly smooth, delimited by a shallow furrow; surface with numerous fine radial riblets which are interrupted by prominent con centric lamellae; hinge quite similar to Eomiodon, provided with small but distinct posterior cardinal tooth 5b, as formulated:AIII AI 3a 3b 5b PI/AII 2 4b PII; lateral teeth not crenulated; ventral margin smooth internally. (adapted from Hayami, 1965b, p.131)
Age and distribution. —Neocomian-Albian. Japan and Formosa.
Remarks. —Costocyrena was regarded as a member of the Corbiculidae in the Treatise, but as originally considered and subsequently supported by Ohta(1973), it is certainly referable to the Neomiodontidae. Costocyrena shares many common internal characters with Eomiodon and was probably derived from that genus.

Pseudasaphis Matsumoto, 1938, Jour. Geol. Soc. Japan, 45 (532): 17.
Systematic position. —A genus of the Neomiodontidae.
Type-species. —Pseudasaphis japonica Matsumoto, 1938 (original designation) [Pl. 7, Fig. 12]
Diagnosis. —Shell medium-sized, trigonally ovate; umbo prosogyrous, anteriorly located; hinge essentially similar to that of Eomiodon and Costocyrena, with distinct posterior cardinal tooth 5b and elongated and non-crenulated lateral teeth; surface ornamented with narrow but strong radial ribs which typically consist of two orders of strength; concentric lamellae undeveloped.
Age and distribution. —Cenomanian. Japan.
Remarks. —Pseudasaphis was included in the Corbiculidae with a query in the Treatise, but Hayami (1965b) and Ohta (1973) suggested that it is referable to the Neomiodont idae owing to the similarity of hinge structure to Eomiodon. It is reasonable to consider that Pseudoasaphis was evolved from Eomiodon through Costocyrena.

Crenotrapezium Hayami, 1958b, Japan. Jour. Geol. Geogr., 29 (1-3): 13.
Systematic position. —A genus of the Neomiodontidae.
Type-species. —Crenotrapezium kurumense Hayami, 1958 (original designation) [Pl. 7, Figs. 13, 14]
Diagnosis. —Shell of medium-size, triangular, with sharp posterior carina; umbo more or less prosogyrous, prominent; escutcheon and lunule not impressed; dentition as formulated: AIII AI 3a 3b PI PIII/AII 2 4b PII; cardinal 5b undeveloped at all; lateral teeth transversely crenulated; surface smooth.
Age and distribution. —Lower Jurassic-Tithonian, (?) Lower Neocomian. Japan.
Remarks. —Although originally proposed as a member of the Arcticidae, this genus, as subsequently regarded by Hayami (1959b) and also in the Treatise, is more appropriately referred to the Neomiodontidae. The carinated outline is somewhat similar to Eotra pezium Doubillé, 1912, but the hinge structure is almost identical with that of Neo miodon.

Mesocorbicula Suzuki and Oyama, 1943, Venus, 12 (3-4): 143, 147.
Systematic position. —A subgenus of Myrene (Neomiodontidae).
Type-species. —Corbicula tetoriensis Kobayashi and Suzuki, 1937 (original designation) [Pl. 8, Fig. 1]
Diagnosis. —Shell subtrigonal to subovate, moderately inflated; lunule limited by a weak ridge; no escutcheon; surface smooth or only with concentric lamellae; dentition formula: AIII AI 3a 3b (5b) PI PIII/AII AO 2 4b PO PII; lateral teeth cross-striated; pallial line entire.
Age and distribution. —Upper Jurassic. Japan.
Remarks. —Suzuki and Oyama (1943) proposed Mesocorbicula as a section of Corbicula with a brief diagnosis, but, as clarified by Ohta (1973), the hinge structure is not of corbiculoid-type. Though Mesocorbicula was once regarded as a synonym of Neomiodon by Hayami (1965b, p. 128) and subsequently as synonymous with Veloritina Meek, 1872, in the Treatise, it is more adequately regarded as a subgenus of Myrene, as con sidered by Ohta (1973).

Explanation of Plate 8 Fig. 1. Myrene (Mesocorbicula) tetoriensis (Kobayashi and Suzuki): type-species of subgenus Mesocorbicula. Paratype (UMUT MM7011), rubber cast from bivalved internal mould, ×1. Loc. Izuki, Izumi (UP. Jurassic or Neocomian, Tetori group). Fig. 2. Tetoria (Tetoria) yokoyamai (Kobayashi and Suzuki): type-species of geaus Tetoria. Holotype (UMUT MM7004), right valve, ×1. Loc. Kurouchi, Furukawa (Up. Jurassic, Tetori group). Fig. 3. Tetoria (Paracorbicula) yoshimoensis Ota. Bivalved internal mould (UMUT MM7992-1), ×1. Loc. Yoshimo, Shimonoseki (Neocomian, Yoshimo formation). Fig. 4. Isodomella shiroiensis (Yabe and Nagao): type-species of genus Isodomella. Rubber cast from left internal mould (UMUT MM7885), ×1.5. Loc. Yoshimo, Shimonoseki (Neocomoian, Yoshimo formation). Fig. 5. Nagaoella. corrugata (Nagao): type-species of genus Nagaoella. Topotype (GK H6708), left valve, ×1. Loc. Hiraiga, Tanohata (Aptian, Miyako group). Tig. 6. The same species. Topotype (GK H6518), right valve, ×1. Loc. ditto. Fig. 7. Neoburmesia iwakiensis Yabe and Sato: type-species of genus Neoburmesia. Topotype (UMUT MM3283), Miyama, Soma (Kimmeridgian, Soma group). Fig. 8. Tetorimya carinata Hayami: type-species of genus Tetorimya. Holotype (UMUT MM3181), right valve of a bivalved specimen, ×1. Loc. Nonomata, Makito (Callovian, Tetori group). Fig. 9. Pholadomya (Bucardiomya) miyamotoi Nagao. Topotype (GK H6547), right valve, ×1.5. Loc. Hiraiga, Tanohata (Aptian, Miyako group). Fig. 10. Goniomya (Goniomya) nonvscripta Tamura. Holotype (UMUT MM3292), left valve, ×1. Loc, Tatenosawa, Soma (Kimmeridgian, Soma group). Fig. 11. Goniomya (Goniomya) subarchiaci Nagao. Left valve (GK H6559), ×1.5 Loc. Hiraiga, Tanohata (Aptian, Miyako group). Fig. 12. Cercomya (Cercomya) gurgitis (Pictet and Campiche). Left valve (GK H6574), ×1. Loc. Hiraiga, Tanohata (Aptian, Miyako group).

Isodomella Kobayashi and Suzuki, 1939, Japan. Jour. Geol.Geogr., 16 (3-4): 219.
Systematic position. —A genus of the Neomiodontidae.
Type-species. —Cyrena shiroiensis Yabe and Nagao, 1926 (original designation) [Pl. 8, Fig. 4]
Diagnosis. —Shell medium-sized, trigonally ovate, rounded in front, obliquely truncated at hind; umbo prominent, anteriorly located; pre-umbonal margin broadly concave to form a large lunule; hinge of (?) astartoid-type with strong, transversely striated cardinal teeth 2, 3b and 4b; lateral teeth elongated but weak; surface smooth except for growth lamellae.
Age and distribution. —Neocomian. Japan.
Remarks. —Kobayashi and Suzuki (1939) regarded Isodomella as a section of Polymesoda Rafinesque, 1820. Some authors erroneously interpreted that Cyrena shiroiensis is a synonym of Cyrena naumanni Neumayr, 1890, and this treatment was adopted also in the Treatise (p. N668). As clarified by Hayami and Nakai (1965), "Cyrena naumanni" is undoubtedly referable to Protocyprina, a member of the Neomiodontidae, and re garded as unrelated to "Cyrena shiroiensis". Isodomella has traditionally been included in the Corbiculidae, but the disposition and transverse striation on the cardinal teeth strongly remind me of the hinge of the Neomiodontidae. Anyhow, a further study is required to decide the systematic position of this genus.

Tetoria Kobayashi and Suzuki, 1937, Japan. Jour. Geol. Geogr., 14 (1-2): 44.
Systematic position. —A genus of the Corbiculidae.
Type-species. —Batissa yokoyamai Kobayashi and Suzuki, 1937 (original designation) [Pl. 8, Fig. 2]
Diagnosis. —Shell medium-sized, suborbicular or subelliptical, a little longer than high, typically not strongly inflated; lunule and escutcheon not impressed; pallial sinus deep; hinge of cyrenoid-type as formulated: AIII AI 3a 1 3b PI PIII/AII 2a 2b 4b PII; an terior cardinals generally connected with anterior laterals; lateral teeth long, curved, faintly crenulated; posterior laterals shorter than anterior ones and separated from cardinals by a wide, flattened intervening space. (emended from Ota, 1965, p. 218)
Age and distribution. —Upper Jurassic-Neocomian. Japan and Formosa.
Remarks. —Tetoria was originally proposed as a section of the genus Batissa, but the difference from the type-species of Batissa, as pointed out by Ota (1965), deserves a generic distinction. The radial threads on the surface of the holotype of Batissa yoko- yamai, which were reported by Kobayashi and Suzuki (1937) and regarded by Ota (1965) as one of the diagnostic characters of Tetoria (s. s.), are not considered as original structure.

Paracorbicula Kobayashi and Suzuki, 1939, Japan. Jour. Geol. Geogr., 16 (3-4): 220.
Systematic position. —A subgenus of Tetoria (Corbiculidae).
Type-species. —Corbicula sanchuensis Yabe and Nagao, 1926 (original designation).
Diagnosis. —Shell suborbicular, strongly inflated; surface marked only with concentric growth lines; hinge with three cardinal teeth and both laterals in each valve; cardinals 1 and 2a rarely bifid; laterals curved and anterior ones long. (emended from Ota, 1965, p. 168)
Age and distribution. —Neocomian and (?) Aptian. Japan and Formosa.
Remarks. —Although Paracorbicula was originally proposed as a section of Corbicula s. s., its generic distinction from Corbicula can be based on the more primitive cardinal teeth and deep pallial sinus as well as the suborbicular outline. On the other hand, as pointed out by Ota (1965), Paracorbicula is essentially similar to Tetoria, and the former is here regarded as a subgenus of the latter. Paracorbicula differs from Tetoria (s. s.) in the more strongly inflated shell.

Nagaoella Hayami, 1965b, Mem. Fac. Sci. Kyushu Univ., [D], 17 (2): 145.
Systematic position. —A genus of the Veneridae.
Type-species. —Dosiniopsis corrugata Nagao, 1934 (original designation) [Pl. 8, Figs. 5, 6]
Diagnosis. —Shell small, subelliptical, strongly inflated; umbo prosogyrous, placed an teriorly; lunule superficial, large but shallow; escutcheon clearly delimited; surface shiny, smooth except for growth-lines and numerous subinternal radial threads; hinge of cyrenoid-type, as formulated: (AIII) AI 3a 1 3b PI/AII 2a 2b 4b PII; 3b thick and bifid; laterals weak; pallial line triangularly sinuate; inner ventral margin finely crenulat- ed in accordance with subinternal radial threads.
Age and distribution. —Aptian-Albian. Japan.

Pulsidis Ota, 1964, Mem. Fac. Sci. Kyushu Univ., [D], 15 (I): 149.
Systematic position. —A genus of the Corbulidae.
Type-spedes.-Pulsidis nagatoensis Ota, 1964 (original designation) [Pl. 10, Fig. 9]
Diagnosis. —Shell small, moderately inflated; right valve larger than left, rostrated posteriorly, with a low ridge extending from umbo to postero-ventral margin; left valve trigonal, less developed posteriorly; surface with flat-topped regular concentric ribs; interior of right valve grooved for the reception of the margin of left valve; right cardinal tooth subtriangular, heavy; left valve possessing relatively broad chondrophore; pallial line forming a nearly right angle at postero-ventral corner, slightly sinuated. (abridged from Ota, 1964, p. 150)
Age and distribution. —Neocomian-Turonian. Japan.

Eoursivivas Ota, 1964, Mem. Fac. Sci. Kyushu Univ., [D], 15 (1): 155.
Systematic position. —A genus of the Corbulidae.
Type-species. —Corbula matsumotoi Hase, 1960 (original designation) [Pl. 10, Figs. 7,8]
Diagnosis. —Shell somewhat large for family, thin, elongated subpyriform; right valve slightly larger than left; umbo anteriorly placed; anterior and middle areas moderately inflated, posterior area narrowed and compressed; surface with a few gentle concentric undulations; inner ventral margin of right valve grooved for the reception of the margin of left; right cardinal tooth moderately heavy; left valve having relatively broad sub triangular chondrophore, which is posteriorly continuous with dorsal margin and separated from it by a shallow groove; pallial line with a small and indistinct sinus. (abridged from Ota, 1964, p. 155)
Age and distribution. —Neocomian. Japan.
Remarks. —The generic name of Eoursivivas was ethymologically derived from Ursirivas Vokes, 1945, also a corbulid genus, but emendation of the original spelling would be unjustified, because it is not considered as due to a typographical error.

Nipponicorbula Ota, 1964, Mem. Fac. Sci. Kyushu Univ., [D], 15 (1): 157.
Systematic position. —A genus of the Corbulidae.
Type-species. —Nipponicorbula mifunensis Ota, 1964 (original designation) [Pl. 10, Fig. 10]
Diagnosis. —Shell small, moderately thick, elongated subtrigonal; a prominent ridge extending from umbo to antero-ventral margin; right valve larger than left, rostrated, with two cord-like keels extending from umbo to posterior periphery; left valve not so posteriorly produced; disk of right valve with strong cancellate sculpture, while left valve has moderately strong concentric ribs and a few slender radial ribs confined to middle part; inner margin of right valve grooved for the reception of left margin; right cardinal tooth large; left valve with a relatively narrow, moderately elongated chondro phore. (abridged from Ota, 1964, p. 157)
Age and distribution. —Cenomanian-Turonian. Japan.

Praecaprotina Yabe and Nagao, 1926a, Sci. Rept. Tohoku Imp. Univ., [2], 7 (4): 21.
Systematic position. —A genus of the Caprotinidae.
Type-species. —Horiopleura yaegashii Yehara, 1920 (original designation) [Pl. 10, Fig. 11]
Diagnosis. —Attached valve irregularly conical; tooth strongly projecting; anterior muscle insertion on extension of cardinal platform on thickened area of shell wall, posterior muscle insertion extending as erect plate from tooth to postero-ventral shell wall. Free valve convex; teeth subequal; elongate anterior muscle insertion extending from cardinal platform along shell wall, separated from it by a shallow groove; posterior muscle insertion on thin vertical plate; large posterior accessory cavity extending from dorsal to ventral border. (abridged from the Treatise, p. N787)
Age and distribution. —Aptian. Japan.

Neoburmesia Yabe and Sato, Proc. Imp. Acad. Tokyo, 18 (5): 251.
Systematic position. —A genus of the Pholadomyidae.
Type-species. —Neoburmesia iwakiensis Yabe and Sato, 1942 (original designation) [Pl. 8, Fig. 7]
Diagnosis. —Shell large, inflated; umbo anteriorly located; surface divided into three areas by strong posterior carina and weak anterior carina; anterior and posterior areas covered with weak concentric ribs; middle area with distinct radial and concentric ribs, tuberculate at their intersections; escutcheon long, narrow and well delimited; posterior gaping distinct; hinge edentulous; no chondrophore. (abridged from Tamura, 1960b, p. 279)
Age and distribution. —Kimmeridgian. Japan.

Tetorimya Hayami, 1959g, Japan. Jour. Geol. Geogr., 30: 159.
Systematic position. —A genus of the Pholadomyidae.
Type-species. —Tetorimya carinata Hayami, 1959 (original designation) [Pl. 8, Fig. 8]
Diagnosis. —Shell equivalve, pyriform or rounded subtrapezoidal, expanded postero ventrally, strongly convex; post-umbonal margin long, nearly straight; ventral margin a little concave in anterior part; surface divided diagonally into antero-ventral and postero-dorsal parts by a striking carina running from umbo to postero-ventral margin; umbo subterminal, rising above hinge-line; valve margin nearly closed; surface smooth; pallial line deeply sinuate; hinge apparently edentulous. (emended from Hayami, 1959g, p. 159)
Age and distribution. —Middle-Upper Jurassic. Japan, Alaska, Siberia, Ural and western Europe.
Remarks. —Some Russian authors described similar shells to Tetorimya carinata under the generic name of Bureiamya Voronetz, 1937. As the result of subsequent type designa tion of Bureiamya by Cox (1969, in Treatise), however, Bureiamya can be regarded as a synonym of Bucardiomya, from which Tetorimya is clearly distinguishable.