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(Gmelin, 1791)

Description (shell):
Shell fragile, almost equivalve, with the left valve just a little more convex than the right. Inequilateral, beaks in front of the midline. Approximately oval in outline, with the posterior dorsal line straight and the posterior margin abruptly truncated and gaping. Periostracum often with agglutinated sand grains, echinoderm spines, etc. Ligament is internal, immediately below and behind the beaks, in an elongated, narrow, longitudinal ridge and with an underlying, supporting, white calcareous plate or lithodesma, which is broadest posteriorly. Shell surface with coarse granulations, these gathered into radiating lines some of which are prominent as radiating ribs; also with concentric undulating ridges, which may be pronounced, particularly in smaller shells. Hinge line is without teeth. Inside of shell is glossy, with the external ribbing showing through. Scars are faint, adductors thin, about equal. Pallial sinus is deep, extending beyond the umbones.

Size:
Up to 38.1 mm in length.

Colour:
White in colour, rather translucent. Periostracum light brown. Interior iridescent.

Animal:
The animal is white with a yellowish tinge. The anterior part of the mantle has a slight fringe of a few projecting points. The siphons are short, fringed at their orifices and having a few filaments round the base of each. The white foot is tongue-shaped, extensible and grooved for the production of a byssus (L. norwegica-animal).

Habitat:
Inhabiting sand and silty sand from just offshore to varying depths.

Distribution:
Distributed from Iceland, the Faroes and northern Norway to the Iberian Peninsula, the Atlantic coast of Morocco, Madeira and the Canary Isles (Distr. L. norwegica).

Lyonsia norwegica