(Olivi, 1792)
Description (shell):
Shell thick, broadly oval to subtriangular, umbones close to midline; right valve convex, enclosing and overlapping left, its ventral margin covered only by a fringe of periostracum from the left valve. Sculpture of coarse concentric grooves and ridges, and a few radiating striae on left valve. Right valve with a single, massive tooth, anterior to a recessed, triangular chondrophore, and single elongate anterior and posterior lateral teeth. Left valve with a shallow chondrophore posterior to a deep socket, no lateral teeth. Adductor scars and pallial line usually clear, pallial sinus very shallow (C. gibba-drawing).
Size:
Up to 15 mm long.
Colour:
Dull white to cream; periostracum coarse, grey-brown, usually worn at the umbones, most conspicuous on left valve. Inner surfaces white or yellowish, sometimes tinted blue.
Animal:
The animal has a comparatively large and thick foot, which it uses for burrowing just below the surface. The shell is drawn out behind to make a cover for the siphons, which are so short as to be only slightly protruded (C. gibba-animal).
Habitat:
In muddy sand and gravel, occasionally on the lower shore, most abundant offshore.
Distribution:
Distributed from Norway south to the Mediterranean and west Africa (Distr. C. gibba). Also found in the enclosed Lake Grevelingen in SW Netherlands.