Oval in outline, girdle rather wide, spiculose; valves moderately elevated, carinated, side slopes nearly straight, apices prominent, giving the valves a beaked appearance. On valves sculpture of fine granulation, closely beset with black dots, except for a narrow area along the posterior margin. Head valve is semicircular, anterior slope straight to slightly concave, with a roundish notch at the top, the posterior margin widely V-shaped. There are always a few concentric growth marks. Intermediate valves are broadly rectangular, anterior margin regularly convex or somewhat sinuate, side margins rounded, posterior margin a little concave at both sides of the well pronounced apex; lateral areas raised (C. septemvalvis-detail).
Up to 22 mm long, 14 mm broad.
Varying from brick red to cinnabar, mostly more or less variegated with creamy white, rarely almost entirely cream coloured, or with shades of green, bright yellow, or orange, always glossy.
From subtidal to a depth of 500 m, frequently on red algae.
All Atlantic coasts of Europe, as far northwards as Bodö in Norway and the Shetland Islands, southwards to the Canary Islands and everywhere in the Mediterranean Sea, as far to the east as Haifa and Alexandria, penetrating even into the Red Sea. It is a rather uncommon species (Distr. C. septemvalvis).