Shell reaching 18-20 mm. (about 0,75 inch) in length, attenuated, imper¬forate and moderately sculptured. Whorls 10, moderately convex, well shouldered and joined. Color a uniform grayish-white. Suture deep, aperture subcircular to ovate and oblique. Lip thickened. Columella short and arched. Axial sculpture consisting of rather numerous costae (11 on the body whorl) which may be strongly developed or nearly absent. These terminate above as a series of rounded points which rise above the suture and are generally impressed upon the whorl above. The canal thus formed con¬sists of a series of rather deep pits between these pointed costae. There is generally a very well developed rounded basal ridge. Generally the costae are not formed below the basal ridge and there may be a fairly deep channel below the ridge which outlines the umbilical area. Occasionally the axial costae persist beyond the basal ridge. Microscopic sculpture consists of an exceedingly fine pitted surface. It appears to be equally well developed both on the costae and in the spaces between them. This sculptured layer is quickly lost in worn specimens. Operculum unknown.

With 10 whorls, this shell grows to 20mm. As it was described by Linneaus, it is the senior shell to its junior partner, O. hotessieriana. It is larger and has distinct shouldering on the tops of the whorls. The 11 costae may be distinct or nearly absent. There is a basal ridge below which the costae do not generally persist. O. crenata is found not only from southern Florida through the Caribbean, but also from the western Mediterranean from France south to the Gold Coast in Africa.

We can detect no differences in the characters exhibited by specimens from the West Indies and those from Europe and Africa. Most of the characters possessed by this species are quite variable, particularly in the development of the axial costae and the basal ridge. The basal ridge may exist as a mere terminating point for the costae or as a rather deep spiral groove below7 which is defined a well rounded ridge. The costae may be produced as low flattened ribs terminating as sutural crenulations or exist only as crenulations at the suture. Very occasionally a varix is produced.
Opalia crenata differs from O. hotesseriana by being larger and, most important, by having the whorls shouldered. In hotesseriana the whorls are not shouldered and the crenulations merge evenly into the whorl above.