Shell. Marine, found in almost all parts of the world, some smooth, others knotted, spined, tuberculated, or undulated; thick, generally pearly, spire sometimes depressed, at others elevated and pointed at the summit, sharp or carinated at its circumference, frequently umbilicated, not always; aperture transversely depressed, the margins not united at the upper part; columella arched and twisted, often projecting forward; operculum horny, thin, with numerous spiral whorls, increasing from the centre to the circumference. Inhabits the European seas. Ninety-nine living species. Eight fossil.

3. Genus Monodonta. Pl. [XI].

Animal. See Trochus.

Shell. This genus appears to be the connecting link between the Trochus and Turbo; distinguished from the former, by an entirely rounded aperture, slightly depressed; from the latter, by the toothlike projecting angle the truncated columella occasions at the base; an operculum. Inhabits the Chinese seas. Thirty-four species.

4. Genus Turbo. Pl. [XI].

Animal. Very similar to that of Trochus, the sides sometimes ornamented with tentacular appendages, varying in number and form; head proboscidiform; tentacula thin and setaceous.

Shell. Depressed conical, or subturreted; sometimes umbilicated, frequently carinated at its circumference; interior pearly; aperture round or depressed, the middle of external edge hollowed; edges rarely joined by a callosity; columella arched, rarely twisted, not truncated at the base; an operculum. Inhabits the Indian and American seas. Fifty-one species.