Abstract
Two new cymatiids were found from the Pliocene and the lower Pleistocene formations in the Japan Sea borderland. One of them, Ranella yasumurai, which is newly described on the specimen from the Pliocene Kuwae Formation in Niigata Prefecture, can be easily discriminated from Fusitriton oregonensis. Another species, Fusitriton izumozakiensis having 11-12 axial ribs is restricted to the Pliocene in Akita and Nagano Prefectures and the lower Pleistocene strata in Niigata Prefecture. Both new species suffered extinction at the end of early Pleistocene. On the other hand, the recent species, Fusitriton oregonensis (Redfield) first appeared at the early Pleistocene in the Japan Sea side of Honshu and is now living in Japan Sea. The causes of extinction of Ranella yasumurai and Fusitriton izumozakiensis may be due to the glacial environmental change, like as other characteristic species of Omma-Manganji fauna.
