Echigoceras sasakii, a new Middle Carboniferous nautilid from the Omi Limestone Group, Central Japan

A new genus and species of the trigonoceratid nautilid, Echigoceras sasakii, is described. Specimens of this cephalopod occur in the Middle Carboniferous (probable late Bashkirian) bioclastic rudstone/grainstone of the Omi Limestone Group, Central Japan. Echigoceras is most closely related to the Early Carboniferous genus Stroboceras, but differs in its strongly curved cyrtoconic shell and siphuncular position near the ventral margin. It is possible that Echigoceras is the descendant of Stroboceras.