Shell medium in size, from 20 to 30 mm. in length, elevated, conical and narrower anteriorly. Front slope straight or slightly concave, posterior slope more or less convex. Apex in trout of the middle, the orifice being situated immediately before it. The orifice is long, narrow and trilobated. sometimes having two small teeth on each side. Externally it is outlined or stained with black: its length is from one-sixth to one-eighth that of the shell. Sculpture consists of from 18 to 20 principal radiating ribs colored oyster-white or light gray, between which there are three smaller ribs, the middle one generally larger and all three colored dark greenish or bluish gray. Numerous growth lines cross the shell, forming nod ides, sometimes strong, sometimes very fine. Margin strongly crenulated, the indentations of the principal ribs being deeper than those of the intermediates, thus causing the indentations of the latter to appear in groups of three. Color pattern of the outside is seen in the crenulations. Inside color polished bluish or greenish gray, the sculpture of the outside showing through as white lines. Internal callus of the orifice narrow, long-oval, truncated and excavated behind. 1 ts color is the same as on the inside and it is surrounded by a black line which is irregularly rayed anteriorly and laterally. Muscle impression distinct.

This well marked species is not closely related to any others in the Western Atlantic. It is characterized by the prominent radial ribs being oyster-white, the intermediate ribs being colored a greenish or bluish grey. It is rather widely distributed but not at all common, living attached to rocks in the lower intertidal zone.

Florida, the Bahama Islands and south through the Antilles and Gulf of Mexico to Trinidad.