Shell of medium size for the genus, solid, umbilicate, nacreous within, suture deeply channeled, whorls rounded, base angulate, nearly flat. Nucleus of 1.5 flattened, singly carinate whorls: postnuclear whorls 3.5. Spiral sculpture on early whorls of 4 beaded cords that gradually take shape on emerging from the nucleus. On the body whorl are 4 beaded cords of nearly equal size, interspaces equal. The suture is laid upon a broader and more projecting basal cord until the final half whorl, when this cord is exposed by the descending suture. Base with 6 prominent beaded cords, one bordering the umbilicus and another descending within. Axial sculpture of fine lamellae over entire surface, beading on the spiral cords produced by the thickening and overlapping of 5-8 layers of lamellae in the direction of growth. Lip greatly thickened, spiral cords of base reflected outward and then inward, when seen inbasal view. Color tan, with darker and lighter areas upon the spiral cords in a radial pattern, area adjacent to umbilicus lighter colored. Height, 4.8, diameter, 6.1 mm.

Arena adusta is closely related to A. lurida Dall, 1913, but differs chiefly in having an angulate base rather than the evenly rounded base of A. lurida. Differences between immature shells of the 2 species are even more pronounced, the juvenile shells of A. adusta are discoidal in shape, while those of A. lurida are rounded. Approximately 50 specimens of A. lurida were collected with the type lot of A. adusta. Nearly all were living; they are somewhat smaller and colored gray or brick red, rather than tan as in A. adusta. Arene lurida has been collected at many of the islands along the southwestern shore of the Gulf of California, while A. adusta is known only from the vicinity of Espiritu Santo Island. The name is a Latin adjective, "brown" or "tanned."

Type Locality: Cove adjoining the west sides of Isla Partida and Espiritu Santo Islands, Baja California, Mexico. 24° 25* N, 110° 25' W, LACM station 66-28, 10 April 1966