Six different types of molluscan assemblages are recognized in the Holocene littoral deposits, accumulated during the Climatic Optimum (5, 000-6, 000y. B. P.), along the Pacific coast of Japan. These assemblages are typically observed in the Holocene embayment deposits of the Paleo-Ohuna Bay in central Honshu (Lat. 35°N). They are found from several embayment deposits distributed between Okhotsk coast of northern Hokkaido (Lat. 45°N.) and western Kyushu (Lat. 33°N.), although their existence and local distribution within each embayment were controlled by topography of the bay and bottom sediments.
The assemblages are composed mainly of the warm water species, intermixed with some cold water elements in the northern areas. The northern limits of the distribution of constituting species were located further north than those of the living species. The northward extension of distribution was remarkable in northern Hokkaido. Along the Okhotsk coast of northern Hokkido, the littoral molluscas, whose present day analogs are living in the area south of north Honshu (Lat 41°N.), were found constituting shell beds in the Holocene deposits, and also found from the shell mound in the Early Jomon era. In this area the minimum temperature of surface water at the Climatic Optimum was estimated to be about 5°C higher than at present when compared with the water temperature of northern Honshu. The southern end of the cold water species distribution is recognized in the Sendai area (Lat. 38°N.), where the warm water assemblages are intermixed with the cold water elements. Marine condition of this area was presumably similar to that observed at present in the area off Choshi, central Japan (Lat. 36°N.).
The distribution of warm water species at the time of Climatic Optimum is shown in Figure 13. Estimated minimum surface temperatures are also shown in this figure. The temperature difference between the Climatic Optimum and the present becomes less conspicuous toward the south.