Fig. 167.—Calyptræa sinensis
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.
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.
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.