Shell is fragile, fat, almost equivalve, the left valve just a little more convex than the right and overlapping it; inequilateral, beaks just behind the midline, directed inwards. The anterior half is broadly oval, the posterior half is drawn out into a projecting spout with a terminal gape. Ligament is internal, with a supporting and enclosing lithodesma, the resilifer a small grooved chondrophore. Sculpture prominent, 20-30 radiating ribs on a background of fine concentric lines, the posterior ribs being the most prominent. Two thin ridges run from the umbones to the posterior margin of the spout. Growth stages are not clear. Right valve with a posterior lateral tooth which fits under a small ventral ridge on the posterior dorsal line of the left valve. No cardinal teeth. Inside of shell with a short dorsal internal ridge posterior to the umbones and the external ribbing showing through. Muscle scars etc., indistinct. Margin crenulate where met by the ribs .
Up to 9.5 mm long.
White, sometimes translucent. Periostracum fawn. Inside of shell white, translucent.
Lives in muddy sand and gravel beyond 18.3 m.
It is widely distributed in the North Atlantic, from the Norwegian Sea to the Azores, the Canary Isles, Madeira and south to Liberia, Ghana and Gabon, and also occurs in the Mediterranean (Distr. C. costellata).