(Linné, 1767)
Description (shell):
Shell thick, elongate, rectangular or rather irregular; umbones close to anterior end, anterior margin rounded, posterior truncate. Sculpture of coarse, concentric ridges, less evident about the umbones, where the shell is usually abraded. Two distinct ridges extend posteriorly from umbo on each valve: in juveniles these bear a few short, coarse tubercles, worn away in older specimens. Right valve with a single cardinal tooth, left valve with two, visible in juveniles only, in older shells these are worn completely away, the hinge line below the umbo and ligament being undulating and thickened. Adductor scars large and distinct, pallial line discontinuous, comprising a series of separate scars (H. arctica-drawing).
Size:
Up to 40 mm long.
Colour:
Dull white; periostracum coarse, yellow-brown. Inner surfaces white.
Animal:
The animal is whitish, tinged with yellow. The siphons are capable of great extension and are covered almost throughout their length by a brown sheath.
Habitat:
Attaches by a byssus, in holes, crevices, or algal holdfasts; also bores into soft calcareous rocks and shells. On the lower shore and well out on to the continental shelf (H. arctica-borings).
Distribution:
Distributed from arctic waters south to, at least, the Mediterranean and north-west Africa (Distr. H. arctica).