Ribbed Ark Shell, Acar gradata

Ribbed Ark Shell, Acar gradata. Size: 2.5 cm (1.0 inch) x 1.3 cm (0.5 inches). Shell collected off the beach El Tule, Km 17, Cero Colorado, Baja California Sur, October 2018. Identification courtesy of Bob Hillis, Ivins, Utah.

The Ribbed Ark, Acar gradata (Broderip and G.B. Sowerby I, 1828), is a bivalve mollusk that is a member of the Arcidae Family of Arks. The shell is small in stature 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. The Ribbed Ark Shell reach a maximum of 6.8 cm (2.7 inches) in length and 3.5 cm (1.4 inches) in height.

Ribbed Arks are found attached to rocks, in the intertidal zone to depths of 37 m (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.