Two-row Rock Shell, Stramonita biserialis

Two-row Rock Shell, Stramonita biserialis. Shells collected off the beach in the greater Los Cabos area, Baja California Sur. Size: 4.7 cm (1.9 inches) x 3.1 cm (1.2 inches). Identification courtesy of Bob Hillis, Ivins, Utah.

Two-row Rock Shells, Stramonita biserialis. Shells collected off the beach for consumption  in the greater Bahía Santa Rosalittita area, Baja California, March 2016. Collections and photographs courtesy of Barry Mastro, Escondido, California.

The Two-row Rock, Stramonita biserialis (Blainville, 1832), is a gastropod mollusk that is a member of the Muricidae Family of Rocks. They are known in Mexico as pu’rpura de Blainville. They derive their name from the double row of knobs that are most prominent at the shoulder. The shell is large, strong and sturdy consisting of six whorls that have fine, dark, spiraling line, a moderately high and pointed spire. The aperture is large with the inner lip being crenulated (sculpted with fine grooves) which may extend well into the interior of the shell. The exterior of the shell is white, with heavy gray to dark brown mottling; the interior is white with varying amounts of apricot, light brown blotching, orange, and yellow. The Two-row Rock Shells reach a maximum of 8.1 cm (3.2 inches) in length and 5.3 cm (2.1 inches) in height.

Two-row Rock Shells are found attached to rocks in the intertidal zone to depths up to 3 m (10 feet).  They range from Cedros Island, Baja California to Chile, including the Galapagos Islands. They are found throughout the Sea of Cortez.

Synonyms Purpura bicostalis, Purpura haematura, Purpura peruviana, Thais biserialis, and Thais peruvensis.