Abstract
Fossil and Recent Macoma (Rexithaerus) of the northwestern Pacific consist of the following three species : Macoma (Rexithaerus) hokkaidoensis sp. nov., M. (R.) sectior Oyama and M. (R.) shiratoriensis (Matsubara). Among them, the first species, which is new, is characterized by an elongate shell shape and a low pallial sinus, and is now living in the sea around Hokkaido, Kunashiri Island, Aniva Bay of Sakhalin, Peter the Great Bay and Ussuri Bay of Primorye. The oldest species, M. (R.) shiratoriensis appeared in subtropical waters in the late early Miocene. The Recent and allied species, M. (R.) sectior and M. (R.) cf. hokkaidoensis first occurred in middle Miocene deposits. From the middle Miocene to early Pleistocene, the subgenus was confined to the mild-to cool-temperate realm. In the Holocene, M. (R.) sectior has extended its range to subtropical waters, while M. (R.) hokkaidoensis now inhabits the cool-temperate to subarctic waters.
