Shell thick and solid, inflated, inequilateral, somewhat transversally elongate in shape and with height greater than inflation, roughly quadrate to trapezoidal in outline; slightly inequivalve, left valve slightly overlapping right valve along posteroventral margin. Anterior margin rounded, ventral margin widely convex and meeting the oblique posterior margin at a blunt angle. Umbones moderately prominent, situated well anterior to midline valves. Posterodorsal slope flattened to slightly concave toward posterior end of dorsal margin, set off by a broad rounded ridge radiating from the umbones to posteroventral end of shell. Cardinal area rather narrow and elongated. About 28 radial ribs (26 to 30) at each valve, as wide as the interstices, mainly granulated on left valve. Periostracum well developed, concentrically striated, scaly to spiky in the interstices of ribs. Internal margins with strong crenulations corresponding with the external radial ribbing. No byssal gape. Colour: outside of shell white, frequently tinged deep bluish green posteriorly; periostracum dark greyish brown. Interior whitish.
