A new species of Arca (Bivalvia: Arcidae) from the lower Miocene Asahi Formation on the Japan Sea side of central Honshu, with remarks on the westward faunal migration from the eastern Pacific

The arcid bivalve, Arca (Arca) budoensis new species, is described from the Budo Mudstone Member of the Asahi Formation in northern Niigata Prefecture, central Honshu. The age of the member has been assigned to 16.6-15.9 Ma (late early Miocene) on the basis of dinoflagellate cysts. Judging from the arcid, dinoflagellate cysts and sedimentary facies, the paleoenvironment of the member was a shallow embayinent influenced by a warm-water current. The reason that the molluscan fauna of the Budo Member contains no characteristic species of the subtropical to tropical Arcid-Potamid fauna (17.0-16.7 Ma) is that the Budo fauna is slightly later than that Arcid-Potamid fauna. Because the new species resembles fossil and Recent species of the eastern Pacific, A. budoensis is accepted as derived from an ancestor that migrated westward to Asia. The senior author, Kazutaka Amano, is the single author of the new species.