Page:Natural History, Mollusca.djvu/221

Rh All the species of the genus feed on marine vegetables: I have watched with pleasure the little Grey Top (T. cinerarius), the commonest species of our weed-clad rocks, rasping down with the teeth of its ribbon-like tongue the minute Confervœ that grow on the inside of my glass vases. With a pocket lens it is easy to see the process, which I can compare to nothing else than the mode in which a cow licks up, as it were, the grass, as she moves along, by successive sweeps of her tongue.

(Keyhole-shells.)

In external form and appearance, the shells which compose this group bear the closest resemblance to the Limpets (Patelladæ). All of them, however, have the peculiarity of an orifice in the shell, either at the summit of the cone, or in the form of a slit at the front edge.

The characters of the included animals distinguish them at once; they have well-developed heads, with short muzzles, and tapering tentacles, at the outer bases of which the eyes stand on short footstalks. Beneath the mantle on each side is a series of short tentacular filaments, similar in character to those of the Trochidæ. There are two gill-plumes, which are large, pectinated, and equal; they are placed in an ample cavity, which communicates with the aperture of the shell, whether this be situated on the summit, or in the front margin.

All the mollusks of this family are marine, and are distributed through the seas of most parts of the world; but principally those of warm climates.