Earlyto Middle Miocene molluscan faunas in southwest Japan and Korea are studied with respect to new data on molluscan assemblages, correlation and paleoposition of Southwest Japan in the viewpoints of paleobiogeography and paleoecology.
Two molluscan faunas are recognized in Early to Middle Miocene sequences in southwest Japan, the lower, of which age is 18+～16 Ma and the upper, 16～15- Ma. The difference between the lower and the upper faunas is represented by the presence of different species in the same genera and by the absence or presence of some species in either the lower or the upper fauna.
The lower fauna named the Akeyo Fauna lacks characteristic species suchas tropical sea species included in the upper fauna. The upper fauna belongs to the so-called Kadonosawa, Fauna and is similar to the molluscan fauna, in Korea.
The so-called Kadonosawa Fauna in southwest Japan and Korea represents strong influence of warm currents from the south in the beginning of the Middle Miocene (16～15Ma) which is also the time of the opening of the Sea of Japan.