(Spengler, 1792)
Description (shell):
Shell thin and fragile. Anterior lobe triangular with fine horizontal ridges; anterior disc with even finer oblique ridges; median disc narrow, concave, with concentric sculpture; posterior disc with concentric lines; auricle smooth, crescent-shaped, much smaller than anterior lobe. Both valves strongly convex. Umbonal reflections small, closely applied to anterior face of umbones; hinge line posterior to umbo broad and concave, delimited by a raised ridge. Median disc, and junction of auricle with posterior disc, evident as distinct ridges. Apophysis broad, bladelike, extending half-way to opposite ventral border. A knoblike condyle below the umbo, and another ventrally. Pallet concave, blade elliptical with a straight edge, handle cylindrical, extending along midline of blade as a distinct ridge.
Size:
Up to 20 mm long.
Colour:
White, periostracum brown, darkening to almost black in larger specimens. Inner surfaces white.
Animal:
Worm-like animal with enormously developed siphons. For the protection of the body a chalky tube is secreted. At the mouth of this tube the animal has developed a pair of accessory valves - pallets - the handles of which are attached to the body. The oval blades of the pallets serve to close up the tube when the siphons have been withdrawn.
Habitat:
Bores into fixed and floating wood.
Distribution:
Widespread in the north-east Atlantic, from Iceland to the Mediterranean (Distr. N. norvegica).