Shell thick and solid, inflated, inequilateral, roughly quadrate in shape with arcuate ventral margin and obliquely truncate posterior margin; slightly inequivalve, left valve distinctly overlapping the right valve ventrally and posteriorly. Cardinal area rather long and narrow. About 33 radial ribs (30 to 36) at each valve; ribs as wide as the interstices, granulated on left valve. Periostracum well developed. Internal margins with strong crenulations corresponding with the external radial ribs. No byssal gape. Colour: outside of shell white under the blackish brown periostracum. Inner side whitish.