(Linné, 1758)
Description (shell):
Shell thick, tumid, irregularly oval or rhomboidal in outline; umbones anterior but subterminal, with a projecting, rounded, anterior margin; dorsal margin markedly convex, ventral margin slightly concave. Umbonal ridges well-developed. Young specimens with numerous long, smooth, slender periostracal spines, frequently with attached detritus, lost as the shell grows, represented only by a roughened area at posterior end. Sculpture of fine concentric lines and ridges, growth stages often clear (M. modiolus-drawing).
Size:
Size variable, commonly 50-100 mm.
Colour:
Shell bluish white to slate-blue, darkening in larger specimens; periostracum very glossy, light horn to deeper mahogany or dark brown in old shells, usually with a lighter yellow-brown strip along the umbonal ridges. Inner surface of shell white, sometimes with a faint bluish tinge towards posterior margin.
Animal:
Colour dark orange; the foot is red, whitish towards the thick and wrinkled base. Both margins of the mantle are without fringe of toothing, though covered with delicate cilia.
Habitat:
Burrows is muddy gravel. Distributed from lower shore to about 150 m; offshore it may form immense aggregations.
Distribution:
In the whole North Sea, south to the Bay of Biscay (Distr. M. modiolus).