Shell large,the left valve being slightly larger and its ventral margin overlapping that of the right valve. Angulated dorsal margin, rounded anterior end and obliquely truncated posterior end, with a small concave depression but without any keel. Umbones elevated, located a little towards the anterior margin and slightly prosogyrate. Surface sculptured with 32 to 35 flat, strong and grapular radiating ribs. The ribs on the left valve coarsely granular and the interstices are slightly narrower than those on the right valve;