A small, thin and smooth shell. The lower valve is always smaller than the upper valve. No ribs, but closely set concentric growth striae.
Up to 10 mm in diameter.
White, grey, cream or yellow, sometimes mottled with brown.
The animal has about 20 unequal ocelli (eye-spots) in two series, black with a pearly centre.
On sand and fine gravel bottoms, 4-1800 m deep.
A cosmopolitan species, but mainly found in the northern hemisphere. From southern Iceland and northern Norway south to Angola. Also reported from the Canaries, Madeira and the Mediterranean (Distr. H. similis ).