Shell very small, rather convex and tumid, softly shining, egg-shaped in outline, with its peaks very close to the posterior margin. Prodissoconch very large, occupying 1/3 of the total length of the shell, smooth, with finely granulated surface. The rest of the surface is covered by fine irregular concentric ribs and some growth lines. Very fine grooves radiating from the beak may be present. The faint pallial line and the adductor scars lie very high on the shell (these may be invisible). A strong ridge present on the upper part of the interior; this starts near the posterior adductor scar, passes the cardinal teeth and ends before reaching the anterior adductor scar. In both valves two cardinal teeth; in left valve the anterior one is poorly developed, posterior rather long; in right valve both cardinal teeth are very strong. In both valves two lateral teeth present.
Up to 1.5 x 1.0 mm.
Fresh shells with broad, irregular, concentric, white colour bands; prodissoconch is colourless and transparent. Worn shells are opaque and white.
Only known in the North Sea from northern parts (Distr. M. dawsoni).