(Linné, 1758)
Description:
Shell very tumid, umbones just in front of midline, prominent, blunt, and enrolled. A thick ridge extending from beaks to anterior and posterior margin on each valve. Cardinal area a slender diamond shape with fine chevron striations, ligament deep brown. Periostracum coarse, brown, forming a distinct fringe ventrally. Sculpture of fine radiating and concentric ridges, giving a decussate effect on the central areas of each valve, but with only radiating ridges prominent at anterior and posterior margins. Growth stages visible as raised ridges. Hinge line with 40-50 teeth. Anterior and posterior margins rounded. Adductor scars well-marked (S. lactea-drawing).
Size:
Up to 20 mm long.
Colour:
Off-white or yellowish; periostracum brown; inner surface white.
Habitat:
From the lower shore to below 100 m, on shell-gravels and coarse bottoms.
Distribution:
In the southern North Sea (Distr. S. lactea). Distributed south to the Mediterranean, West Africa, and the coasts of South Africa.