Shell subquadrate, inflated. Sculpture of heavy, broad, flattened, widely spaced radial ribs, and very fine commarginal striae. Left valve slightly overlapping right valve. Periostracum thick, with sharply pointed setae. Length to 125 mm.
Ephemerally as far north as Anacapa Island (34.0°N) [LACM] and Palos Verdes (33.7°N) [LACM], California, and not common on the Pacific coast of Baja California north of Laguna Ojo de Liebre [Scammon's], Baja California Sur (27.8°N) [CASJ. within the Golfo de California, and south to the Golfo de Panama (9°N) [CAS, USNM] and the Islas Galapagos (1.0°S) [CAS], from the intertidal zone to 230 m. Reported as early as the Pliocene in western North America.