Shell normally almost circular; depressed conical; white or livid, sometimes tinged by the growth of a green or pink nullipore. The upper surface marked with from thirty to fifty radiating striae or riblets which are reticulated by the coarse, somewhat elevated lines of growth. The margin in young shells is denticulated, but with age this character is lost. The interior is usually polished, except on the muscular impressions, which are granulous and strongly impressed. The scar is horseshoe-shaped, the right anterior limb broad near its extremity. Close to the anterior extremity of the left limb is a small oval scar indicating an attachment of the mantle of the shell. The apex is posteriorly curved, subcentral, smooth, and in the youngest specimens I have seen exhibits no tendency to spiral growth. In older specimens it is usually somewhat eroded. The texture of the shell is exceedingly solid, porcelainous, and strong. Old individuals fill up the cavity of the apex while adding to the margin. From the irregularities of their stations most fantastic forms occur. (Dall.)

Type locality, Valparaiso.
RANGE.Trinidad and the Farallon Islands to Chile.