The genus Nuttallia can be distinguished from other psammobiids by having a wide hinge plate, a subumbonal pit and the lower limb of the pallial sinus detached from the pallial line. It consists of the Nuttallia petri and N. nuttallii groups, based on the outline and size of shell. Among the large-sized N. petri group, Nuttallia commoda can be separated from the extant species N. petri by having a thin-walled shell, a compressed right valve, a gently sloping dorsal margin, and a lower shell. Moreover, it has been elucidated that N. commoda is one of the characteristic species of the Omma-Manganji fauna, which was confined to the Pliocene and lower Pleistocene deposits in the Japan Sea borderland. From the fossil records, the N. petri group appeared earlier than the N. nuttallii group geologically. Probably it evolved from the Eocene Soletellina, which spread northward taking advantage of the warm climate during the Eocene and adapted to the cooler temperature of the Oligocene in the northwestern Pacific. On the other hand, the N. nuttallii group flourished in the northeastern Pacific during the Miocene to Pliocene and migrated to the northwestern Pacific by the Pliocene.
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Vol. 16 • No. 2