The Cup-and-saucer Limpet (_Calyptræa sinensis_) is so called on account of a curved plate that projects from the interior of the shell, at the apex; and though this plate takes the form of a half-cup rather than of a cup, the whole shell has suggested the popular name, while the generic name is derived from _calyptra_, which signifies a cap. This mollusc is occasionally found among stones at low tide, but usually lives beyond this line, thus necessitating the use of a dredge. The Bonnet Limpet (_Pileopsis hungaricus_) is of similar structure and habit, but the nucleus of the shell is a more decided spiral (see Plate V.). Both these animals adhere to stones and rocks, and, like the common limpet, seldom or never move from their selected sites; hence their shells are variable in form, being adapted to the rock below, and the movements of the shell often cause a little hollow to be scooped out of the softer materials. [Illustration: FIG. 167.--_Calyptræa sinensis_] Yet other limpets belong to the next family _Fissurellidæ_, which is characterised by a perforation or a notch in the shell. In these, too, the shell is conical, with a tendency to assume the spiral form, but the curve of the nucleus, which is always apparent in the young shell, frequently disappears as the growth proceeds. [Illustration: FIG. 168.--_Fissurellidæ_ 1. _Puncturella noachina._ 2. _Emarginula reticulata._ 3. _Fissurella reticulata_] In the Keyhole Limpet (_Fissurella reticulata_) which is found chiefly on our southern shores, the perforation is at the summit of the shell; but as the animal grows the hole increases in size, encroaching on the curved nucleus until the latter quite disappears. In the genus _Puncturella_ the perforation is just in front of the recurved apex, and is surrounded by a rim internally; while in the Notched Limpets (genus _Emarginula_) it is represented by a fissure on the anterior _margin_ of the cone. In all, however, the hole or notch serves the same purpose, for it is the means by which water enters the siphon. It is doubtful whether we ought to claim the beautiful Ear shell (_Haliotis tuberculata_) as one of our own, but it is generally included among the British molluscs on the ground that it is abundant on the coast of the Channel Islands, where it is called the Omar; and it is certainly too beautiful an object to be excluded from the British species without ample cause. [Illustration: FIG. 169.--_Haliotis_]