Out of a total of 93 sampling stations in the Hamana-ko area, 70 stations yielded ostracods. The samples from the remaining 23 stations contained no ostracods. A total of 45 genera and 70 species were found. Living and dead specimens were distinguished, and a number of species from each station are listed in tables 2 through 5.

Distribution of ostracod species in the Hamana-ko area is dependent on the characteristics of the water (O₂ content, Eh, pH, salinity, etc.) and the substrate and bottom topography of the bay. Based on the characteristic species, 6 major biofacies and 7 sub-biofacies were recognized in the Hamana-ko area:
I. Fresh water pond and river biofacies
II. Brackish water inlet biofacies
III. Bay coast and sand bank biofacies
IV. Inner bay biofacies
(1) Dy sub-biofacies
(2) Sandy bottom sub-viofacies
(3) Silty bottom sub-biofacies
(4) Silty sand bottom sub-biofacies
V. Channel and extension channel biofacies
(1) Main channel sub-biofacies
(2) Extension channel sub-biofacies
VI. Bay mouth and open sea biofacies
(1) Bay mouth sub-biofaces
(2) Open coast sub-biofacies
(3) Offshore sub-biofacies

Distribution of these ostracod biofacies and sub-biofacies is shown in text-figure 3.

I. Fresh water pond and river biofacies: Many small ponds for breeding eels are found around the Hamana-ko. The number of fresh water ostracods in the ponds varies seasonally. Bottom mud of the ponds was obtained at stations 63, 64, and 76. Candonocypris assimilis, the dominant species of this biofacies, makes up 90-100% of the total number of specimens. Other subordinate members of this biofacies are such fresh water inhabitants as Potamocypris producta and Cypridopsis vidua and some slightly brackish water species of Physocypria sp.

River mouths are in general occupied dominantly by one or more of the abovementioned four fresh water species derived from eel-breeding ponds. In addition to these species, fresh water Ilyocypris angulata and Stenocypris major (which are characteristic in running water), a subordinate new species of Cyprididae from the brackish inlet biofacies, and Mutilus assimilis, Xestoleberis hanaii from the bay coast biofacies are present. At stations 71, 80, and 90, the assemblage, consisting of three fresh water species-Candonocypris assimilis, Potamocypris producta and Cypridopsis viduacharacteristic of eel-breeding ponds, and the new species of Cyprididae, dominant in brackish water environments, is different from the assemblage in eel-breeding ponds in the domination of Potamocypris producta and Physocypria sp. over Candonocypris assimilis and the presence of brackish water species. The assemblage in station 22 is also characterized by fresh water species from the eel-breeding ponds with some bay coast species, Mutilus assimilis and Xestoleberis hanaii. Although stations 74 and 75 are located close to the mouth of the bay, such fresh water species as Ilyocypris angulata, Candonocypris assimilis, Stenocypris major, and Cypridopsis vidua bear a very high ratio to the marine water species: a ratio ranging from 28 to 100%. Inference of the direct supply of abundant fresh water species from an adjacent eel-breeding pond to the nearby rigorous marine environment may be supported by the paucity of live specimens and the abundance of the dead carapaces. In conclusion, the minor environmental condition in this fresh water biofacies seems to be strongly controlled by the condition of the water, i.e., salinity, agitation, etc., and the effect of the substratum; grain size distribution of sediment is likely to be quite insignificant.

II. Brackish water inlet biofacies: The coastline of Hamana-ko, characterized by drowned valley topography, has several small inlets with very narrow mouths. In the entire area of the inlets with narrow mouths and the inner half of the inlets with wide mouths, a brackish water environment is produced by the limited circulation of marine water. These areas are occupied exclusively by the new species of Cyprididae. Examples can be seen in stations 2, 3, 19, 45, 68, 70, 72, 85, 86, 87, 88, and 89. In stations 5 and 84 where partial growth of Zostera can be observed, Cytherois zosterae appears subordinately to the dominant new cypridid species. In Station 82, where the effect of the fresh water is significant, a small amount of dead carapaces of Ilyocypris angulata, Candonocypris assimilis and Cypridopsis vidua are found. On the other hand, a certain amount of Xestoleberis cf. dentata and X. hanaii derived from bay coast biofacies are also found at stations 67 and 73. In conclusion, brackish water inlet biofacies are found on clay or silt substrata partially overgrown with Zostera and are characterized by monotonous ostracod assemblage dominated by the new cypridid species.

III. Bay coast and sandbank biofacies: This biofacies is found in three areas: a narrow sand zone along the rocky coast of the main body of the bay excluding the inlets, a narrow sand zone along the northern edge of the sand flat, and the wide sand bank area just inside the mouth of the bay. In these areas, circulation of the fresh oceanic water with high salinity, entering through the narrow mouth of the Hamana-ko and running along the channel leading deeply into the inner half of the bay, has a strong influence on the nature of the substratum as represented by coarse sand and gravel. The bottom is commonly covered by various kinds of algae. Characteristic species of this biofacies are Mutilus assimilis, Aurila hataii, Xestoleberis spp. and Loxoconcha spp. Mutilus assimilis dominates in stations 15, 25, 26 and 28, all of which are located in the area where the channel biofacies terminates. Hemicytherura tricarinata and Semicytherura? miurensis derived from channel biofacies constitute the subordinate members in these stations. Typical examples can be found at stations 25 and 28 on the northern edge of the wide sand flat of the central part of the bay. In stations 15 and 26 of the narrow zones along the rocky coast of the main body of the Hamana-ko, where algal growth is noticeable, Xestoleberis spp., Cytherois zosterae, and Paradoxostoma japonicum can be added to the members of subordinate species. At stations 35 and 38 in the wide area just inside the mouth of the bay, Aurila hataii, Xestoleberis setouchiensis Okubo, Mutilus assimilis, and Loxoconcha (L.) pulchra are found in nearly equal numbers. Besides the four dominant species, Pontocythere spp., Cythere lutea omotenipponica, Callistocythere spp., and Neonesidea oligodentata, derived from the open sea biofacies, occur as subordinate species. The distribution of Neonesidea oligodentata is limited to the area close to the mouth of the bay. Stations 10, 20, 21, and 66 are characterized by the dominance of Xestoleberis spp. Subordinate members consist of Hemicytherura tricarinata and Semicytherura? miurensis from channel biofacies. A small amount of Paradoxostoma japonicum and Cytherois zosterae from the algal habitat and some fresh water species are also found. These stations are located at the terminal area of the channel biofacies. In conclusion, bay coast and sand bank biofacies of Hamana-ko are an extension of the ostracod biofacies characteristic of the tide pool, rocky shore, and nearby coarse sand found along the open sea coast of the Pacific.

IV. Inner bay biofacies: Biofacies of the main part of the bay and inlets can be subdivided into four sub-biofacies, which correlate significantly with the nature of substratum.

(1) Dy sub-biofacies: As explained in the preceding chapter, in the area of black silt and clay with abundant humus, the deficiency of dissolved oxygen and production of toxic hydrogen sulphide in summer does not permit inhabitation by animals, including ostracods. An exceptional case was found only in station 12 where a few live specimens of Hemicytherura. tricarinata occur in this sub-biofacies. Ostracods found in this biofacies consist of dead and more-or-less corroded carapaces of such species as Hemicytherura tricarinata, Semicytherura? miurensis, Cytheromopha acupunctata, Mutilus assimilis, and Spinileberis quadriaculeata from the adjacent biofacies. No ostracods are found in the dy from stations 1, 4, 6, 7, 9, 11, 14, 16, 17, 18, 24, 30, 31, and 81.

(2) Sandy bottom sub-biofacies: The central part of the main bay is occupied by a wide flat area with very well-sorted sand. Rapid sedimentation of sand carried from the open sea by a tidal current characterizes this area (Ikeya and Handa, 1972). Ostracods are extremely poor in the number of individuals as well as in the number of species. A small number of Spinileberis quadriaculeata, Semicytherura? miurensis and Cytheromorpha acupunctata are found at stations 33 and 34. Other species, a new cypridid species and Physocypria sp. seems to be derived from the adjacent biofacies.

(3) Silty bottom sub-biofacies: This sub-biofacies occupies the main part of the largest inlet of the Hamana-ko bay called Inohana-ko, The flat silt bottom is in water approximately 2 m deep; the water has a relatively low salinity (ca. 23‰) and is in-habited by dominant species of Spinileberis quadriaculeata, as exemplified by stations 44, 49, and 50. A subordinate species of this sub-biofacies is Cytheromorpha acupunctata; other species occur only rarely.

(4) Silty sand bottom sub-biofacies: In the southern part of the Inohana-ko inlet, a channel extending in the northeast direction separates the area into two: an area of northwestern silty bottom sub-biofacies, as explained above, and an area of the southeastern silty sand bottom sub-biofacies. The latter sub-biofacies is found on the bottom, which is coarser in grain size and higher in salinity (ca. 26‰) than the silty bottom of the former sub-biofacies. In this sub-biofacies, dominant and subordinate species relative to the silty bottom sub-biofacies become reversed and Cytheromorpha acupunctata becomes the dominant species, and Spinileberis quadriaculeata becomes the subordinate species. Hemicytherura tricarinata and Semicytherura? miurensis are rarely found.

V. Channel and its extension channel biofacies: Biofacies on the bottom of channels can be divided into two sub-biofacies: one corresponds to the water course of the inflow of marine water, and the other can be seen in the areas of spreading of the marine water running through the channel over a flat sandy bottom.

(1) Main channel sub-biofacies: Marine water flows into the main part of the bay through its narrow mouth, runs northward, and then branches into northwest and northeast flowing streams. The former stream again branches into two streams in the inner area of the bay: one stream runs further northward, and the other runs eastwards along the northern flanks of the wide central sand flat. Marine water runs toward the Inohana-ko inlet through two channels joined together at the entrance of the inlet and runs further north along the eastern shore of the inlet. Biofacies of the bottom of these channels is characterized by Hemicytherua tricarinata and Semicytherura? miurensis, both of which occur in nearly the same numbers and predominate by far over other species. In detail, however, in localities near the mouth of the bay where the influence of the marine water is prominent-as exemplified by stations 27, 40, 42, and 43-the former species outnumbers the latter species, whereas at stations 8, 10, 13, 23, 27, 29, and 32, which are located in the relatively inner area of the bay, the latter species predominate over the former species. Appearance of subordinate species also varies, reflecting the slight environmental difference. The occurrence of Loxo-concha spp. and Clithrocytheridea japonica characterizes main channel sub-biofacies near the mouth of the bay, whereas Mutilus assimilis appears in the same sub-biofacies of the inner bay area. It is interesting to note that even though the number of individuals is very small, species from the other biofacies from fresh water to open sea get mixed in this sub-biofacies.

(2) Extension channel sub-biofacies: This sub-biofacies is found in the area close to the bay mouth and occupies the low ground where the main channel sub-biofacies extends into the bay coast and sand bank biofacies. Dominant species are Clithro cytheridea japonica and Hemicytherura tricarinata. Semicytherura? miurensis, dominant in the main channel sub-biofacies, becomes subordinate. Loxoconcha spp., Xestoleberis spp. and Aurila hataii, characteristic of the bay coast and sand bank biofacies, are also found in this sub-biofacies.

VI. Bay mouth and open sea biofacies: Three sub-biofacies which correspond well to the differences in substratum are distinguishable in the area from the mouth of the bay toward the open sea.

(1) Bay mouth sub-biofacies: This sub-biofacies is distributed from just inside to just outside the mouth of the bay called Ima-gire, where the swift stream of tidal current makes the bottom exclusively of the gravelly substratum. Three species, Platy microcythere tokiokai, P. sp. and Cytheroma? sp., dominate in this sub-biofacies. In the area inside the mouth exemplified by station 39, the order of dominance is in descending order: Platymicrocythere sp., Cytheroma? sp., and Platymicrocythere tokiokai; in the area just outside the mouth exemplified by station 61, the order is Cytheroma? sp., Platymicrocythere sp., and P. tokiokai; and in the area farther outside the mouth exemplified by station 60, Platymicrocythere tokiokai predominates over Cytneroma? sp. These three extremely small species are found alive and perhaps live in the inter stitial water between gravels. The large number of subordinate species makes the species diversity of this sub-biofaices the highest among all the biofacies.

(2) Open coast sub-biofacies: Runoff material from the Tenryu River is trans ported westwards along the coast of Enshu-nada Sea by the coastal current; its medium sand fraction is deposited near the shore and fine sand fraction offshore. Medium sand found along the open sea coast of 0-10 m in water depth is occupied mainly by Pontocythere spp. Callistocythere spp., which are characteristic among many subor dinate species. Examples can be seen in stations 52, 54, 55, 56, 57, and 58. Considerably high species diversity is also found in this sub-biofacies.

(3) Offshore sub-biofacies: Offshore sub-biofacies occupies an extensive area of fine sand, 10-20 m in water depth, Outside the area of open coast sub-biofacies. In this sub-biofacies, the order of dominance in the open coast sub-biofacies becomes reversed and Callistocythere spp. predominate over Pontocythere spp. Species subor dinate in number of individuals but characteristic of this sub-biofacies are Cytherel loidea munechikai, Neonesidea oligodentata, Neocytherideis punctata n. sp. and Munseyella japonica. All of these species are representatives of the shallow warm water inhabitants of the open sea of the Kuroshio region. Altogether, these species bring species diversity up to a very high level. The areal distribution of fourteen characteristic species is shown in text-figures 4 through 7.

It is interesting to note that some species which have their counterpart species in tropical and subtropical area (e.g., Trachyleberis sp., Actinocythereis sp., and Neocytheretta sp.) are represented only by young molts in the Hamana-ko area; these species are not rare and are found in the upper Pleistocene to Recent warm water sediments in Japan (e.g., Aburatsubo Cove and upper Pleistocene deposits in Chiba Prefecture). Adult forms are, however, extremely rare; in fact, adults of some species have not yet been found in Japan. The explanation may be the difference of temperatures necessary for survival and for reproduction of these species, as suggested by Hazel (1970) in the discussion of temperature control of ostracod distribution. Environment suitable for reproduction of these species may be restricted to small areas, and the mortality rate of young molts may be comparatively high in the Hamana-ko area.