(Linnaeus, 1758)
Description (shell):
Shell is brittle and inequivalve, right valve flat, left valve strongly convex, overlapping the right. Inequilateral, beaks are in the anterior half. Crescent-like in outline, the posterior dorsal line concave, anterior dorsal line convex sweeping into a long curved ventral line and a sharp posterior truncation; the posterior dorsal line of the left valve is inset and the gap left is filled in by a lateral projection along the length of the corresponding line in the right valve. Both valves may be slightly twisted posteriorly. Ligament is internal, in a resilifer which is a long narrow groove on the inside of each valve directed obliquely and posteriorly from below the beaks. The borders of this groove are raised and in specimens in which the ligament has been lost they may appear as isolated ridges or lamellae. In front of the resilifer in the right valve a long vertical tooth-like ridge projects, fitting into a notch in the umbo of the left valve, and a small groove just beneath it. No lithodesma. Sculpture is weak, of fine concentric lines and undulations; two faint, smooth, almost parallel, curved ridges run from the umbo of the left valve posteriorly. Inside of shell the anterior dorsal line is thickened in the left valve. Adductor scars are round, almost equal. Pallial line is represented by a series of separate, small, muscle scars. There is no partial sinus.
Size:
Up to 38.1 mm in length.
Colour:
White, sometimes faintly yellow. Periostracum is light brown. Inside of shell is pearly white, tinted green.
Animal:
The animals have a single gill on each side, the outer fold of which is prolonged over the back. The mantle margins are united almost throughout. The siphons are united almost to their full length and their orifices are fringed. The blunt beak of the shell behind protects the lengthened siphons (P. albida-animal).
Habitat:
Inhabits sand and sandy mud, from very low water-mark to medium depths. On very sheltered beaches it may occur in abundance.
Distribution:
Only in the southern North Sea (Distr. P. albida).