Abstract
The Lower Jurassic Higuchi Group, exposed in the western part of Shimane Prefecture, Japan, includes, in ascending order, the Orojidani and Higuchidani formations. The Orojidani Formation is composed of conglomerate, sandstone, and mudstone, the latter containing Oxytoma sp. and “Pleuromya” sp. The Higuchidani Formation consists mainly of dark gray mudstone, from which six genera and six species of bivalves were collected: Kolymonectes staeschei, Palmoxytoma cygnipes, Ryderia texturata, Pseudomytiloides matsumotoi, Oxytoma sp., and Pleuromya sp. This bivalve assemblage is characterized by Boreal elements such as K. staeschei and P. cygnipes, which are known exclusively from the Lower Jurassic of Russia and northern Canada; it contains no typical Tethyan species.
