Shell thin and brittle, oval, umbones just anterior to midline. Sculpture of very fine concentric lines, growth stages clear. Shell margin prominent at hinge line: lunule and escutcheon absent. Right valve with two cardinal teeth, the anterior parallel to hinge line, and paired, elongate anterior and posterior laterals. Left valve with three cardinal teeth, anterior two joined to form a single, widely forked structure, the third poorly developed, and single elongate anterior and posterior laterals. Chondrophore triangular, posterior to the cardinal teeth in each valve, with a small dorsal septum isolating it from the external ligament. Adductor scars and pallial line indistinct; pallial sinus broad and rounded, not extending far into shell.
Up to 50 mm long.
White, tinted purple about the umbones, with light brown rays of varying width radiating from umbones; periostracum light brown, thin. Inner surfaces glossy, white, tinted purple.
Colour white tinged with blue. The animal has a thick mantle fringed with white filaments. The siphons are short, their mouths fringed with yellow or red filaments; united throughout their length, but the exhalent tube is fitted with a tubular valve. The foot is large, tongue shaped, and very extensible, used for burrowing and leaping.
Burrowing in clean sand, from the lower shore into the shallow sublittoral.
Widespread, and often abundant (Distr. M. stultorum). Distributed from Norway to the Mediterranean and West Africa.