(Schumacher, 1817)
Description (shell):
Shell solid, equivalve; almost equilateral, beaks just in front of the midline, directed inwards and slightly forwards, the umbones are often eroded. Variable in outline from almost circular to broadly oval. Ligament is prominent, reaching almost half the length of the escutcheon. Sculpture of numerous incised concentric lines; sometimes concentric ridges may appear about the umbones. Growth stages
obscured. Right valve with two cardinal teeth of which the anterior is very broad and the posterior thin. Left valve with three cardinal teeth of which the posterior is very small and inconspicuous. Margin is smooth.
Size:
Up to 4.5 cm in length.
Colour:
White. Periostracum very darkbrown to almost black in adults; light brown in
juveniles.
Animal:
Body almost round, flattened from the sides. The lips are large; the mantle is thick, not fringed with tentacles, but part of it forms a short excretory siphon behind. The foot is conical and small, though powerful.
Habitat:
From 2 to about 3000 m deep.
Distribution:
The species is distributed from the Arctic south to Japan, the Aleutians, Massachusetts Bay and the northern North Sea (Distr. A. borealis).