The breathing apparatus in this small group consists of a great number of little conical leaflets, arranged in a circle, more or less complete around the body, and attached beneath the margin of the mantle. The animal is covered by a shell varying much in dimensions and in structure in the different genera. They are all somewhat sluggish animals, adhering for many hours together to the surface of rocks, or other bodies, by means of the foot, which is large and muscular. All of the species inhabit the sea.
A conical shell is the distinctive character of this family; showing no trace of a spire, and destitute of any aperture or notch, by which other genera are known, which have shells of similar form and appearance. The shell, which is made out of one entire piece, quite covers the body; it is in the form of a widened cone, the apex of which is higher or lower, nearly central, or more approaching one end,—in different species. The animal is large in proportion to the shell, with a mantle, under the projecting edge of which, is a fringe of small leaves, that perform the office of respiration. The head is furnished with a large but short proboscis, and with two pointed tentacles, each of which carries an eye at the outer side of