Crassadoma pusio

(Linné, 1758)

The shell is thick and irregularly oval. Inequivalve: right valve with a small byssal notch in anterior ear, cemented to the substratum in later life, becoming irregular, and more deeply convex than the left, upper valve. Small individuals, attached only by the byssus, with 35-50 regular, radiating ribs on each valve; larger, cemented shells with up to 70 coarse, spiny ribs on each valve, usually worn or partly obscured by encrusting organisms. Margin is crenulate (C. pusio-drawing).

Up to 50 mm long.

Variable, from dull white or grey, to yellow, red or brown, sometimes in irregular patterns.

Mantle with a double margin, the inner finely fringed and the outer edged with long tentacles. The eye-spots at the base of the tentacles are few in number.

Adult stage lives attached to hard substrata by its lower valve. From low on the shore, to at least 100 m.

Distributed from Norway and Iceland, south to west Africa, and in the Mediterranean (Distr. C. pusio).