Shell is fragile, flat, equivalve with a smooth margin. Inequilateral, beaks just behind the midline, directed inwards. Outline of shell is broadly oval with the dorsal margins curved and ventral margin almost straight. Ligament is internal, below and behind the beaks. Sculpture of very fine concentric lines and a few microscopically small pits puncturing the shell surface about the umbones. Growth stages are clear. Each valve with a small central cardinal tooth. Right valve with two anterior and two posterior laterals; left valve with one anterior and one posterior lateral. Muscle scars are indistinct, adductors about equal. Pallial line is not indented by a sinus (H. nitidum-drawing).
Up to 3.2 mm in length.
Dirty white, sometimes translucent. Periostracum light yellow. Interior of shell is white, sometimes translucent.
The mantle which is fringed with long tentacles, protrudes nearly all round, and the foot can be flattened in such a manner that it resembles the foot of a snail and it is used in similar fashion. Foot is bluish, flecked with white; extends in front to form a siphon.
Gravelly and stony sand, offshore between 15-165 m.
All around the British Isles, the Norwegian Sea and Baltic south to the Iberian Peninsula, the Bay of Tangiers and into the Mediterranean (Distr. H. nitidum).