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.
Up to 20 mm long.
White, periostracum brown, darkening to almost black in larger specimens. Inner surfaces white.
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 The oval blades of the pallets serve to close up the tube when the siphons have been withdrawn.
Bores into fixed and floating wood.
Widespread in the north-east Atlantic, from Iceland to the Mediterranean (Distr. N. norvegica).