Shell is solid and inequivalve: left (lower) valve is convex, right (upper) valve tends to be flat, though often bent sitting within the left. Inequilateral, tending to be broadly oval in outline but often distorted and irregular. Beaks and umbones are not prominent. Ligament is internal, attached to a central triangular pit, with lateral extensions. Sculpture of concentric ridges and lines with a few irregular radiating ribs on the left valve which do not normally meet or indent the margin. Hinge line is without teeth in the adult. Adductor muscle scar is near the posterior margin, a deep purple or red-brown in colour. Shell margins are smooth (C. virginica-inside).
Up to 178 mm from hinge-line to the opposite margin.
White, dirty white or brown, sometimes with dark purple markings. Periostracum is thin, dark brown. Interior of shell is white.
Native of the coast of North America where it is distributed from the St. Lawrence to the Gulf of Mexico and Panama. Although imported in British Isles before 1939 it does not appear to have become established (Distr. C. virginica).