832. CRETACEOUS EOMIODON AND COSTOCYRENA (BIVALVIA) FROM SOUTHWEST JAPAN

Abstract

In this paper 4 species of Eomiodon and 8 species and 2 subspecies of Costocyrena (including Costocyrena ohnishii, sp. nov., Costocyrena hojiensis, sp. nov. and Costocyrena otsukai obsoleta, subsp. nov.) belonging to the Family Neomiodontidae are described from the Cretaceous at various localities in southwest Japan. Geographic distribution of Eomiodon and Costocyrena species seems to change through the Cretaceous time in southwest Japan. In the Lower Cretaceous, Eomiodon species restrictedly occur from the southern side of Kurosegawa Tectonic Belt, while all Costocyrena species are known from the northern side of that belt, Northern Chichibu Belt. In the Upper Cretaceous, the geographic distribution of the two genera becomes overlap, and they occur together even at the same localities especially in central Kyushu. This may not mean habitat change of these bivalves, but seems to support the hypothesis of the Early Cretaceous movement of a large lateral fault in the Chichibu Belt.