Shell usually pure white and brilliantly polished within, but occasionally with a faint yellowish or pinkish flush when very deeply colored externally. Inner margin, except on the hinge-line, always pure white. Outside, covered with a thin yellowish green or olive, epidermis usually wanting, color yellowish white, with rose-pink and pure white rays, color stronger on the lines of growth. Sculpture consisting of incised lines radiating from the umbones where they become obsolete; crossed by rounded, smooth, concentric ridges rather sharply defined by concentric grooves. These ridges are usually regular but sometimes bifurcating. In perfect specimens the radiating lines and grooves are so sharp that their intersections appear as if punctured. Ligament pit deep and excavated, cardinal and lateral teeth moderate. Anterior end much produced, rounded, margin rounded below, posterior end very short, almost truncated; shell rather inflated. Lunule deeply impressed, narrow, lanceolate, short. General form subquadrate. Umbones inconspicuous, usually tinged with yellow. Hinge-line below the lunule with a purple spot. Interior marked with extremely faint radiating lines. Posterior portion of the inferior margin produced. Shell thick and solid. Alt. 1,35 in. Lon. 1,55 in. Diam. 0,7 in. Lunule 0,24 inch.
Habitat. Beach at Soquel, Monterey Bay, two or three valves, Dall; San Pedro, Cooper ; Neah Bay, one worn valve, Swan.
I hardly know with what species to compare this very distinct form. It is perhaps nearest to S. flavicans, Gould, from Panama, which wants the red lines, is more rounded and higher, with a shorter lunule, and much narrower and smaller pallial sinus, besides many other differences.
It has not the peculiar spongy surface of decisa, which is a very high round shell, with the interior strongly punctate and valves nearly equilateral. It wants the curious sculpture, color, and form of S. rupium, which has also a rounder shell and smaller mantle bend. It bears no resemblance to S. incongrua, Cpr., and has not the sculpture or painting of S. pulchra.