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(Montagu, 1803)

Description (shell):
Shell thin and fragile, broadly oval, tending to be irregular. Anterior and posterior margins somewhat angled (compared to Thyasira sarsi ). Hinge line concave anteriorly, convex posteriorly; umbones on midline, beaks clearly directed anteriorly. Lunule heart-shaped, shallow; escutcheon elongate, well-marked, hinge line pouting. Sculpture of fine concentric lines, growth stages visible but not conspicuous. Smooth ridge extends postero-ventrally from umbones to margin of each valve, marked on either side by distinct groove; lower groove more conspicuous and causes deep indentation where it reaches posterior margin. Hinge teeth absent, although hinge line of right valve may be thickened below umbo, forming short callus, or peg. Pallial line broad, ill-defined, adductor scars indistinct (T. flexuosa-drawing).

Size:
Up to 18 mm long.

Colour:
Dull white, with pale yellowish brown periostracum. Inner surface white.

Animal:
The thickened margin of the mantle is open all round and not prolonged into siphons. The long slender foot is almost cylindrical.

Habitat:
Offshore, to the edge of the continental shelf, in muddy sand.

Distribution:
Not in the southernmost part of the North Sea (Distr. T. flexuosa); distributed from Norway to the Azores, extends into the Mediterranean.

Thyasira flexuosa