The Tertiary Tainohata Formation of the Kôbe Group has yielded 10 species of Gastropoda and 19 species of Bivalvia. The formation is of late Middle Eocene age on the basis of the dinoflagellate cyst assemblage and radiometric data. The occurrences of the genera Trinacria, Vicarya, and Sulcobuccinum strongly support an Eocene age for the Tainohata Formation. The molluscan fauna shows high endemism at the species level, whereas it is characterized by Tethyan—Indo-West Pacific genera. Two indigenous molluscan assemblages, Potamides-Cyclina and Corbicula, were discriminated. The composition of these assemblages compares with those of the Neogene intertidal assemblages in embayments. This fact indicates that “temporal parallelism” in the shallow embayment assemblages can be traced back to the late Middle Eocene age in the Japanese Islands.

The following 10 new species are proposed herein: Potamides huzitai sp. nov., Cerithideopsilla hondai sp. nov., Crepidula tainohataensis sp. nov., Cantharus (s.l.) andoi sp. nov., Barbatia nodal sp. nov., Trinacria nipponica sp. nov., Trapezium (Neotrapezium) kobe sp. nov., Tellina (Gastranopsis?) sumaensis sp. nov., Corbicula (Corbicula) uejii sp. nov., and Pitar ozakii sp. nov.