Page:Natural History, Mollusca.djvu/218

206 often covered and disguised by irregular incrustations of calcareous matter, deposited by the water, which may serve as a protection to the animals, causing them to take the appearance of rough stones or masses of earth. This species, like all its fellows, displays very little of its body when crawling.

An extensive group of very fine shells is included under the above appellation, many of which are of considerable size, of very regular and elegant shape, and of exquisite beauty of colour, and sometimes of sculpture. Our own shores possess many species, among which are some of the finest of our univalve shells, and specimens distinguished by all of these characteristics. Yet the finest species are, as usual, exotic, and tropical; for the great Pearly Top (Trochus pica) of the West Indies, the Imperial Sun (Imperator imperialis) of Australia, and the Perspective Staircase (Solarium perspedivum) of the Indian Seas, belong to this family. The shell in this large group varies considerably in form, but is always spiral; the spire sometimes is drawn out to great length, at other times so much depressed as to be nearly flat; but it always forms a large portion of the shell. The aperture is entire, without notch or canal, as the animals are destitute of a siphon.

The animal has a head terminating in a broad muzzle, and often ornamented with head-lobes; side-lobes greatly developed, and furnished with