(Dillwyn, 1817)
Description (shell):
Shell is solid, equivalve; almost equilateral, beaks in the midline. Very broadly oval and almost circular in outline but somewhat variable in this feature. Ligament reaching almost half the length of the escutcheon. Sculpture of forty to forty-five concentric ridges. Lunule and escutcheon are present, each with fine radiating lines. Growth stages are not clear. Right valve with two cardinal teeth, the anterior broad and bifid, the posterior thin. Left valve with three cardinal teeth of which the two anterior are prominent and the posterior very inconspicuous. Growth stages are clear. Margin is smooth (A. montagui-drawing).
Size:
Up to 1.3 cm in length.
Colour:
White. Periostracum with irregular radiating rows of microscopically fine pits; light brown to very dark brown in colour.
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:
Lives in clean sand, mud and sandy gravel offshore to about 73 m.
Distribution:
Collected in the North Sea from about the Dogger Bank northwards and off the north and west coasts of Scotland (Distr. A. montagui). It is distributed from Greenland across the Arctic, through the Kara Sea and the Siberian Ice Sea to the Bering Straits and south to the Bay of Biscay, the Aleutians and Nova Scotia.