It belongs to the family _Haliotidæ_, and our illustration will show that the shell is less elevated than that of limpets, and that the spire, though not prominent, is a fairly well-formed spiral. All along the outer lip of the very large aperture is a series of perforations, occupying the summit of a prominent, spiral ridge, and becoming gradually smaller and smaller towards the spire. The whole shell is pearly in structure, and displays a great variety of rich colouring. It is used largely for inlaying and other ornamental purposes, and for making the so-called pearl buttons. The animal is used largely as an article of food in the Channel Islands, but it is of so tough a nature that it requires a vigorous beating previously to being cooked. [Illustration: FIG. 170.--_Ianthina fragilis_] The same family contains the beautiful violet _Ianthina_, which also is not a British species, but a free-swimming oceanic snail. It is, however, occasionally drifted to our shores, though generally in an imperfect condition. In the Atlantic and the Mediterranean it sometimes abounds in such multitudes as to distinctly colour the surface of the sea. It will be seen that the shell is round, with a well-formed spiral. The spire is white, but the base is of a deep violet colour. The animal is very remarkable in some respects. In the first place, though it has pedicels similar to those on which the eyes of the higher univalves are placed, yet it has no eyes. Then the foot, which is in itself small, secretes a float or raft so large that it cannot be retracted into the shell, with numerous air vesicles to render it light, and the egg-capsules of the animal are attached to the underside of this. The animal has no power of sinking, but lives exclusively at the surface; and, when disturbed, it exudes a violet fluid that colours the surrounding water. It is apparently the only gasteropod that lives in the open sea and has a large and well-formed spiral shell. Passing now to the family _Turbinidæ_ we meet with turbinated or pyramidal shells that are of a brilliant pearly lustre within, and frequently without also when the epidermis is removed. The animals inhabiting them have well-formed heads with a short muzzle, long and slender tentacles, and eyes mounted on peduncles. The sides are ornamented with fringed lobes and several tentacle-like filaments, and the aperture of the shell is closed, when the animal is retracted, by a spiral operculum. They are all vegetable feeders; and, as is usual with the plant-eating molluscs, the teeth on the lateral portions of the lingual ribbon are very numerous.