(Lesueur, 1817)
Among the largest species of atlantids, attaining a diameter of 10 mm. Entire adult shell composed of conchiolin. With growth, adult shell overgrows calcareous larval shell and keel eventually surrounds the last shell whorl. Truncate anterior edge of keel ends at shell aperture. Protoconch with prominent, evenly-spaced, zigzag spiral ridges that cover the surface. Shell spire involute (two whorls of larval shell helicoid). Operculum triangular to trapezoidal and lacks a spiral part. Only one-half of the shell opening (aperture) covered by operculum when animal retracts into shell. Type c eyes, with large pigmented base, which reaches beyond the distal part of the lens. Radula very large and triangular in shape. Median tooth tricuspid; in juvenile specimens with three pointed cusps, in adult specimens with one central cusp flanked by blunt flattened lobes. Lateral teeth with a strong process on the inner side, bearing a pointed accessory cusp. Marginal teeth monocuspid.
See the family Atlantidae for a discussion of the species groups.