Shell is thin, fragile and oval. Dorsal margin is downcurved, tapering posteriorly. Umbones and umbonal ridges are very prominent, giving the two valves a very tumid appearance. Anterior margin is almost straight. With 15-18 broad ribs anteriorly, 20-35 posteriorly. Fine concentric lines present, most conspicuous on the prodissoconch. External sculpture is visible on inner surface (M. subpicta-drawing).
Up to 20 mm long.
Shell off-white to light yellow-brown, periostracum horn-coloured, or light green, occasionally with darker brown or red-brown markings. Inner surface of shell white.
On rocky shores. Occurs on the lower shore and in the shallow sublittoral, among algal holdfasts, attached to undersides of shells and stones, or frequently embedded in the tests of large tunicates, particularly Ascidia mentula and Ascidiella aspersa.
Widespread and common (Distr. M. subpicta). Present in Mediterranean.