Shell thick and solid, but often brittle and flaking easily when dead. Shape variable: typically oval or pear-shaped, or circular when young, becoming irregular, freely influenced by substratum and often distorted. Left valve often deeply cupped, with coarse concentric sculpture, frequently developed as flat scales, and crenulated bands marking growth stages; numerous irregular, radiating ribs give a crenulated effect to margin; surface often abraded, or encrusted. Right valve usually more brittle than left, with well-marked concentric sculpture but lacking ribs. Adductor scar elliptical or comma-shaped, smooth-surfaced, white or yellowish white (O. edulis-inside).
Up to 100 mm long, rarely larger.
Colour off-white, yellowish, or cream, with light brown or bluish concentric bands on right valve. Inner surface of both valves pearly, white or bluish grey, often with darker blue areas marking inclosures of detritus.
At early stage of existence the left valve becomes cemented to a stone, an older shell or some other object on the bottom. The animal has no foot, because it is not needed for locomotion or for spinning a byssus. The mantle has a double margin with a fine fringe. The lips are large and nearly triangular. The solitary muscle for closing the valves is placed nearly in the centre of the shell. Left on their natural banks the animals are full-grown in about four years, but when dredged and laid down again in the culture beds they take several years longer. High commercial value as edible delicacy. (O. edulis-animal).
From the lower shore to about 80 m.
Widely distributed around the British Isles, as indigenous populations or introduced stocks. Occurs naturally from Norway south to the Mediterranean (Distr. O. edulis).