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(Montagu, 1803)

Description (shell):
Shell is brittle, inflated, inequivalve, right valve is more convex than the left. The umbo of the left valve punctures that of the right. Inequilateral, umbones are in front of the midline. Somewhat irregular and often very distorted in outline, but generally varying between triangular to elongate or broadly oval, often with an anterior dent or incision in the ventral margin of the valves. Ligament external and internal; external ligament is short and fat behind the beaks; internal ligament in a semi-circular resilifer below this, with a small lithodesma (often lost) in front of it. Sculpture of irregular concentric lines and undulations on a coarsely granulated surface. Growth stages sometimes clear. Hinge is without teeth. Anterior adductor scar is long and thin, posterior scar is triangular and thicker. Pallial sinus is shallow.

Size:
Up to 25.4 mm.

Colour:
Exterior and interior of shell are white. Periostracum is light yellow or fawn.

Habitat:
Lives in crevices and holes in rocks, from low in the intertidal zone to medium depths.

Distribution:
Distributed from northern Norway south to the Iberian Peninsula, into the Mediterranean and along the Atlantic coast of Morocco (Distr. T. distorta).

Thracia distorta