The buttonsnail, Modulus modulus, is a small marine gastropod of the family Modulidae. The shell is top-shaped and low-spired, and is wider than it is high, consisting of 3-4 or 5-6 strongly convex and angulate whorls. The body whorl is disproportionately larger then the other whorls. The margin of each whorl has a ridge or keel formed by a prominent spiral cord and parallel to 3-4 weaker spiral cords. The aperture is nearly round and the lip is moderately thin and crenulate (serrate-scalloped). The columella terminates in a deep notch that accommodates the pallial tentacles of the inhalant siphon. The shell color is usually yellowish white, splotched with purple or brown, although this is often obscured by the periostracum and algal epiphytes in living specimens. The colummela has a purple tinge and the columellar notch has a purple spot. The operculum is round, thin, and horny (Houbrick 1980, Abbot and Morris 1995).The living animal is light green to mossy green in overall appearance. The head is equipped with a short, bilobe-tipped snout and two thin tentacles. The fully exposed foot is slightly smaller than the shell diameter (Houbrick 1980).