i) STUDIES ON TRIASSIC BIVALVIA

Paleontological studies on Triassic Bivalvia from Japan were commenced by European scholars (Mojsisovics, 1888; Diener, 1915) with description of some species of Daonella and Monotis from Shikoku and Kitakami mountains together with ammonites. These works were followed by some Japanese paleontologists (Matsushita, 1926; Yabe and Shimizu, 1927; Yehara, 1927; Kobayashi, 1931, 1935), who reported the occurrence of some characteristic Triassic bivalves from. Kitakami, Shikoku and west Honshu, and contributed to the age determination of various Mesozoic formations. The discovery of a peculiar trigoniid, Minetrigonia, from west Honshu (Saeki, 1925; Kobayashi and Katayama, 1938) is also worthy of note. Monographs of Halobia and Daonella from Japan were published by Kobayashi and Aoti (1943) and Kobayashi and Tokuyama (1959) respectively.

The development of descriptive studies on the Triassic bivalve faunas reached its climax in the 1950's. Prolific Carnian-Norian bivalves including Trigonucula, Tosapecten and Sakawanella as new genera, called Kochigatani fauna, were known to be widely distributed in the Kochigatani group and its correlatives along the Outer Zone of South-west Japan and its eastern extension in Kwanto mountains (Kobayashi and Ichikawa, 1949a-c, 1950a-c; Ichikawa, 1949, 1950,1951, 1954b-d; Tamura, 1959a,b; Ozawa and Hayami, 1969). Carnian bivalve faunas characterized by the common occurrence of Tosapecten, Asoella, Minetrigonia, Palaeopharus, Cardinioides and early representatives of Oxytoma and Cardinia were also known in the Inner Zone of Southwest Japan, especially from the Nabae group and its correlatives (Kobayashi and Ichikawa, 1951, 1952a; Nakazawa, 1952, 1954, 1955, 1956), lower part of the Nariwa group (Kobayashi and Ichikawa, 1952b; Nakano, 1957b) and Mine group (Amano, 1955; Tokuyama, 1958, 1959a, b, 1960a-c).

The superfarmily Pteriacea are of special importance in Triassic paleontology and biostratigraphy for their significant adaptive radiation and wide geographical distribution. A comprehensive systematic study on Claraia, Eumorphotis, Oxytoma, Monotis and Cassianella was produced by Ichikawa (1958) on the basis of European type material. Upper Triassic biostratigraphic studies using various species and infraspecific taxa of Monotis (Entomonotis) were attempted by Ichikawa (1950) in Kitakami mountains, by Nakazawa (1963, 1964a) in west Honshu and Kitakami and also by Tamura (1965) in Kyushu. The Bakevelliidae are well represented in the Triassic of Japan, and their classification was discussed in some detail by Nakazawa (1954, 1959) and Tokuyama (1959a).

Special studies on the Myophoriidae were undertaken by Kambe (1951, 1957), Nakazawa (1960) and Tamura (1972). Skytian bivalves from Southwest Japan, including some cosmopolitan species, were collectively described by Nakazawa (1953, 1961, 1971) and those from limestone fades by various authors (Ozaki and Shikama, 1954; Ichikawa and Yabe, 1955; Yabe, 1956; Kambe, 1963). Murata (1973) reported some bivalves of this stage also from Kitakami. In addition, Nakazawa (1964b) reported some Anisian bivalves from Shikoku. Mojsisovics' (1888) and Matsushita's (1926) classical works were revised by Ichikawa (1963) and Nakazawa (1971) respectively.

Recent progress of conodont biostratigraphy proved that the Chichibu terrain previously regarded as Upper Paleozoic is in part actually Triassic. Although recent descriptive studies are rather rare, occurrence of Monotis and some other characteristic Triassic bivalves were reported at various placed in this terrain, endorsing the age estimation.

About 190 specific and 40 infraspecific names have been applied for the classification of Japanese Triassic Bivalvia, but I feel some groups have been grossly oversplit. Most of the varieties and formae hitherto proposed do not seem to constitute distinct taxa and are therefore unnecessary. As pointed out by some Japanese and Russian authors (Kiparisova et al., 1966; etc.), the Triassic bivalve faunas from Japan, particularly Carnian and Norian ones, are intimately related to those of eastern Siberia, and further comparative studies may be needed to clarify the paleobiogeography. Generally speaking, Skytian, Carnian and Norian bivalves are well represented in Japan, but Anisian and Ladinian bivalves seem to be rare except for some species of Daonella. Rhaetian fauna has not been reported in Japan and its adjacent areas.

ii) STUDIES ON JURASSIC BIVALVIA

Paleontology on Jurassic Bivalvia from Japan was commenced by Yokoyama (1904) with description of six species from the Lower Jurassic Shizukawa group in Kitakami mountains. By the end of the Second World War, however, little had been attempted on the study of Jurassic bivalves except for several sporadic studies on a few species of trigoniids (Yehara, 1921, 1927), inoceramids (Kobayashi, 1926), cyrenoids (Kobayashi and Suzuki, 1937) and Neoburmesia, a peculiar pholadomyid (Yabe and Sato, 1942).

After the war, Kobayashi and his collaborators examined Jurassic trigoniids and describe dmany species from various localities in Honshu and Shikoku under a new scheme of classification, which basically followed Cox' (1952) but included proposal of several new subfamilies and such characteristic genera as Geratrigonia, Latitrigonia and Ibotrigonia (Kobayashi and Kaseno, 1947; Kobayashi, 1954, 1956a, 1957a, c; Kobayashi and Mori, 1954, 1955; Kobayashi and Tamura, 1955, 1957). The stratigraphical significance of these trigoniids was summarized in the conclusive part of their serial study (Kobayashi, Mori and Tamura, 1959). Maeda and his collaborators carried out further studies on the Jurassic Trigoniidae, describing several new species from the Upper Jurassic of central Japan (Maeda, 1962a, c, d, 1963b; Maeda and Kawahe, 1963, 1966a; Maeda and Adachi, 1965).

On the other hand, the description of rich Upper Jurassic ordinary marine bivalves from the Torinosu group in the Outer Zone of Southwest Japan and from the Soma group (upper part) in Abukuma mountains of north Honshu was commenced by Kimura (1951, 1956) and fully accomplished by Tamura (1959c-f, 1960a-d). Limestones predominate in these groups, and Somapecten, Somapteria, Somarctica and Neoburmesia are characteristic elements of this fauna.

I engaged myself in the descriptive study on Jurassic marine and brackish-water bivalves from various sedimentary areas in the Inner Zone of Southwest Japan and Kitakami mountains, namely, Lower Jurassic Kuruma group and Upper Jurassic Tetori group (lower part) in Hida mountains, Lower Jurassic Shizukawa, Middle-Upper Jurassic Hashiura, Karakuwa, Ojika and some other groups in Kitakami mountains and Lower Jurassic Toyora group in west Honshu (Hayami, 1957a-e, 1958a-d, 1959a-h, 1961a,b, 1969; Hayami, Sugita and Magumo, 1960). Radulonectites, Crenotrapessium, Yokoyamaina, Kobayashites and Tetorimya were proposed as new genera in the course of this faunal study. Classification of Cardinia and Jurassic inoceramids was discussed in some detail (Hayami, 1958e, 1960b), though it is already out of date. Hitherto obtained knowledge on the Jurassic bivalves from Japan was summarized in connection with Jurassic stratigraphy and paleogeography (Hayami, 1961c). The taxonomic position of Upper Jurassic neomiodontids, which had once been discussed by Suzuki and Oyama (1943), was recently restudied by Ohta (1973) toegther with some Neocomian species.

In spite of the narrowness of distributed areas, the Jurassic formations of Japan bear various bivalve faunas in accordance with the remarkable change of sedimentary facies and biogeographic provinces, and about 230 species have been distinguished. As noted before (Hayami, 1961c), Jurassic molluscan faunas of Japan seem to have been highly localized. Common elements are comparatively rare between different sedimentary areas but for some widely distributed Tethyan species in the Upper Jurassic Torinosu fauna and a few cosmopolitan or Siberian elements in the Lower Jurassic. It is also worthy of note that some Tithonian marine species survived until Berriasian, as indicated by associated ammonites, and that a more significant faunal change can be expected in the midst of Neocomian rather than at the Jurassic-Cretaceous boundary.

iii) STUDIES ON CRETACEOUS BIVALVIA

Aside from Schmidt's (1873) description of some Upper Cretaceous mollusks from north Saghalin, Neumayr, Yokoyama and Jimbo are early pioneers in Cretaceous molluscan paleontology in the Japanese islands. Various kind of fossils were collectively treated in their monographs. Neumayr in Naumann and Neumayr (1890) described "Cyrena naumanni" and two allied neomiodontids from the Neocomian of Shikoku. "Avicula haradae" and some other bivalves from the Lower Cretaceous of Kwanto mountains, Inoceramus naumanni and some other important species from the Upper Cretaceous of Hokkaido, and "Trigonia kikuchiana" and a few other characteristic trigoniids from the Lower Cretaceous of Shikoku were reported by Yokoyama (1890, 1891). Jimbo's (1894) work dealt with several inoceramids and trigoniids from the Upper Cretaceous of Hokkaido. The illustrations in these classical monographs were mostly reproduced later with revised specific names and other taxonomic remarks (Hayami, Matsumoto and Asano, 1963; Matsumoto, Hayami and Asano, 1963; Matsumoto, 1963).

Yehara (1915, 1921, 1923a, b, 1927) greatly contributed to the description of Cretaceous trigoniids from Japan, especially from Hokkaido (Ikushumbetsu area), Kitakami (Miyako area), Shikoku (Sakawa and Monobegawa areas) and Kyushu (Amakusa islands). Some pachyodont bivalves from Kitakami and Hokkaido were described by Yehara (1920), Yabe and Nagao (1926) and Nagao (1932b, 1933). Praecaprotina Yabe and Nagao, 1926, is the first proposed Mesozoic bivalve genus on the basis of Japanese material.

During the 1920's and 1930's, ordinary Cretaceous bivalves were vigorously described by Yabe, Nagao and their collaborators: monographs on Upper Cretaceous species from south Saghalin and Hokkaido (Yabe and Nagao, 1925, 1928; Nagao, 1932a, 1938; Nagao and Otatume, 1938), from Amakusa islands of Kyushu (Nagao, 1930; Matsumoto, 1938) and on Lower Cretaceous species from Kwanto mountains (Yabe, Nagao and Shimizu, 1926) and from Miyako area of Kitakami (Nagao, 1934) are important contributions. Most of these papers dealt with gastropods together with bivalves.

On the other hand, Trigonioides and other non-marine molluscs mainly from the Inner Zone of Southwest Japan and south Korea were investigated by Kobayashi and Suzuki (1936, 1939) and Suzuki (1940, 1941, 1943), and finally summarized by Suzuki (1949). Various species of Inoceramus and allied fossils, which are of special importance in Upper Cretaceous biostratigraphy, were in part described by Yabe (1915) and Yehara (1924), and a monographic study including proposal of many new taxa and a classification system was published by Nagao and Matsumoto (1939, 1940).

Thus, many important Cretaceous bivalves from Japan were already described and named by the beginning of the Second World War. In many cases, however, proposal of new taxa was not accompanied by clear designation of type specimens, and variation was often disregarded. Hence a more modernized treatment and nomenclatorial revision became necessary after these works.

After the war, a great number of the studies have been devoted to the description and classification of the Cretaceous Trigoniidae (Kobayashi, 1954, 1957c; Kobayashi and Amano, 1955; Kobayashi and Nakano, 1957, 1958; Nakano, 1957c, 1958a, b; etc.). The stratigraphic occurrence and systematics of the Japanese Cretaceous Trigoniidae were once summarized by Nakano (1960), but a number of subsequent descriptive and taxonomic studies were published on this group (Nakano, 1961a, c; Nakano and Numano, 1961; Maeda, 1962c; Maeda and Kitamura, 1964; Maeda and Kawabe, 1967a,b; Tamura and Tashiro, 1972; Tashiro, 1972).

Additional description and taxonomic refinement of some species of Cretaceous Inoceramus were attempted mainly by Matsumoto and his collaborators (Matsumoto 1957; Matsumoto and Ueda, 1962; Takai and Matsumoto, 1961; Matsumoto and Harada, 1965; Matsumoto and Noda, 1968; Noda, 1971, 1974). A peculiar noetiid, Matsumotoa, was established by Okada (1958) and subsequently restudied by Tamura, Tashiro and Motojima (1968). The unique morphology of Trigonioides, Nippononaia, Plicatounio and some other genera from the non-marine Cretaceous of East Asia has attracted attention of not only Japanese but also European and American paleontologists. Many species were described and disputed opinions were sometimes presented as to the classification and phylogenetical relationship (Cox, 1955; Kobayashi, 1956, 1968; Ota, 1959a-c, 1963; Maeda, 1962b, e, 1963a; Hayami and Ichikawa, 1965; Tamura, 1970; Yang, 1974). Special taxonomic studies were carried out on some Japanese Cretaceous species of Acila (Nagao and Huzioka, 1941), Pholadomya (Nagao, 1943), Opis (Ueda, 1963), corbulids (Ota, 1964), corbiculids (Ota, 1965), neomiodontids (Hayami and Nakai, 1965; Ohta, 1973), Glycymeris (Tashiro. 1971), Anthonya (Tamura and Packard, 1972) and Meekia (Tamura, 1973).

In addition, a number of stratigraphers and paleontologists were engaged in the description of local bivalve faunas, through which many new species and genera were proposed. Some bivalves were described from various Cretaceous strata in south Shikoku (Amano, 1956, 1957b; Katto and Hattori, 1964; Hayami and Kawasawa, 1967), the Albian and Upper Cretaceous in Amakusa islands of Kyushu (Amano, 1956, 1957a, etc.), the Upper Cretaceous in Nakaminato and Futaba areas of north Honshu (Saito, 1962), the Lower Cretaceous in Kamaishi area of north Honshu (Nakazawa and Murata, 1966) and the Lower Cretaceous in Choshi area of Kwanto (Shikama and Suzuki, 1972.) The rich Campanian and Maestrichtian bivalve fauna from. the Izumi group in Izumi mountains and Awaji island of central Japan seems to be important for the general con-sideration of bivalve systematics, since it bears such peculiar genera as Pleurogrammatodon, Micronectes, Izumicardia and Izumia (Ichikawa and Maeda, 1958a, b, 1963, 1966). In the same serial study, taxonomic refinement of some previously described species-was attempted.

More than 130 Lower Cretaceous marine bivalves from Japan except for trigoniids and rudistids were comprehensively described and hitherto obtained knowledge about the bivalve faunas of this age was summarized by Hayami (1965a, b, 1966). In this study, two subfamilies, Pterinellinae and Eomiodontinae, a genus, Costocyrena, and several subgenera were newly proposed, and the classification of Neithea and the Neomiodontidae was discussed in some detail.

About 350 specific and 20 infraspecific names have thus been applied in these descriptive studies. Some of them, especially the early described species and varieties, need further examination in the light of modern taxonomic principles and nomenclature, and many studies are still in progress. The occurrence of marine Cretaceous bivalves is restricted to Northeast Japan and the Outer Zone of Southwest Japan, whereas non-marine bivalves are found primarily in the Inner Zone of Southwest Japan. This con-trast may indicate the paleogeography in this period. Upper Aptian and lower Albian marine bivalves as well as other fossils from the calcareous sediments of the Miyako group in north Honshu are especially abundant and well preserved (Nagao, 1934; Ha-yami, 1965a, b, 1966; etc.). Some of them are closely allied, if not strictly identical, to Tethyan and European (particularly Lower Greensand) species. Several pachyodont species of this stage strongly remind us of the Urgonian fauna in the Mediterranean region. On the contrary, the Upper Cretaceous bivalve characterized by the abundant occurrence of Inoceramus in fine-grained sediments and by trigoniids and other thick-shelled species in coarse-grained ones contain some elements of northern Pacific faunal province.

iv) OTHER STUDIES ON MESOZOIC BIVALVIA FROM JAPAN

Almost all the works mentioned above are concerned with description of local bivalve faunas or classification of some particular taxonomic groups. The ontogeny of some species, though only post-neanic transformation, and the phylogeny of a few families, such as the Trigoniidae and the Inoceramidae, were studied on the basis of Japanese material. The stratigraphic applications and biogeographic significances were discussed in some of these papers.

Although unfavorable states of preservation and other limitations often prevent us from promoting more advanced studies, especially quantitative treatment of fossils, some analytical studies have been attempted in recent years. The Jurassic paleobiogeography was clarified to some extent by using similarity indices between local bivalve faunas; as a result, several faunal provinces were recognized in early and late Jurassic times (Ha-yami, 1961c, 1962a). Tokuyama (1960b) and Tamura (1961b) discussed the bio-facies and litho-facies of the Carnian Mine group and the Upper Jurassic Torinosu group respectively on the basis of the distribution and assemblages of bivalve faunas. Bernard and Munier-Chalmas' dentition formula, which seems to be quite useful for the con-sideration of the classification and phylogeny of Mesozoic heterodont bivalves, was reviewed and discussed (Hayami, 1962b). Kobayashi (1956b) and some others extended the use of this expression to the Trigonioididae. Hayami and Nakano (1968), though provisionally, discussed the application of numerical taxonomy in paleontology, taking the Trigoniidae as an example.

Mesozoic Bivalvia comprise many characteristic extinct genera and families, some of which are quite different from any known Cenozoic and Recent ones in the shell mor-phology, mode of occurrence and probably also in mode of life. Some students are now interested in their functional morphology. Of these extinct bivalves, such thin-shelled groups as the Posidoniidae, Halobiidae, Monotidae and Inoceramidae are particularly unique and interesting for their peculiar morphology, episodic and exclusive occurrence and wide geographic distribution. Studies on the mode of life of these distinguished bivalves were briefly reviewed, and nekto- or pseudo-planktic mode of life was sug-gested for some genera (Hayami, 1969b), though the documentation was insuffcient. Matsumoto and Noda (1968) also suggested planktic mode of life for some species of Inoceramus on the occasion of describing a new species. Tanabe (1973) examined the morphological transformation, particularly the appearance of divergent ribs on the surface, through the ontogeny and phylogeny in the stock of Inoceramus naumanni from the Santonian-Campanian of Hokkaido. As the result of his biometrical study, it was con-cluded that the increase of shell thickness (more precisely, thickness of prismatic calcite layer) to the shell size is negatively allometric before the first appearance of divergent ribs and thereafter becomes positively allometric. He interpreted this phenomenon as indicating the possibility of the evolution of the mode of life from pseudo-planktic to benthonic. As to the Trigoniidae, the living position of Nipponitrigonia and Pterotrigonia was inferred by Nakano (1970c) from their shell morphology and mode of occurrence in the Cretaceous strata of Shikoku. But little has been published about the ecology of other Mesozoic bivalves on the basis of Japanese materials.