Ribbed Ark

Ribbed Ark, Arca gradata

Ribbed Ark, Arca gradata. Size: 2.5 cm (1.0 inch) x 1.3 cm (0.5 inches). Identification courtesy of Bob Hillis.

The Ribbed Ark, Arca gradata (Broderip and G.B. Sowerby I, 1828), is an Ark Shell of the Arcidae Family, that is a small shell with both valves being of equal shape (equivalve) with an elongated oval outline. They have a sloped posterior, a low beak, and a straight, serrate margin. The outside is marked with numerous, coarse, radial, lines, crossed by finer concentric ridges. The shells are white. Ribbed Arks reach a maximum length of 4.1 cm (1.6 inches).

Ribbed Arks are found attached to rocks, in the intertidal zone to depths of 120 feet. They range from Southern California to Peru, including the Cocos, Galapagos, and Revillagigedo Islands. In the Sea of Cortez they are found from Puertecitos, Baja California south to Cabo San Lucas, Baja California Sur.