Few of our shell-fish are more common, in inlets and bays near the mouths of rivers, than these. In such situations they are usually found immersed at the depth of two or three inches in the sand, the place of each being marked by a small, circular, depressed spot. When they open their shells, the entrance into them is protected by a soft membrane, which entirely closes up the front, except in two places, at each of which there is a small, yellow, and fringed tube; by means of which they receive and eject the water which conveys to their body the nutriment necessary for their support.
Cockles are in great request as food among the labouring classes, and are caught chiefly in the winter months. Their size varies from five or six inches to half an inch in diameter. The shell is generally white; it has twenty-six longitudinal ridges, is transversely wrinkled, and has somewhat imbricated striæ. The foot of these animals is largely developed, and is to them a most important organ, as they use it not merely for progression, but in the excavation of hollows in the sand or mud in which they dwell.
The Chama, which is akin to the cockle, was used by the ancients to engrave various figures upon, from which circumstance those small bas-reliefs, so valued now, have obtained among the Italians and collectors the name of Cameos. The shells of some of these are decorated with red or yellow stripes, diverging from the hinge, and spreading to the edges. The Giant Chama has been found to weigh more than five hundred pounds, and the oyster-like animal within was large enough to furnish a meal for twenty men. The animals which inhabit these shells are sometimes called Clams. The shells are often used in Catholic countries for containing holy water.
THE PHOLAS. (Pholas dactylus.)
This is a shell of a rather elongated form, gaping at both ends, and terminated in front by a point; it is white and chalky in its appearance, and the anterior end is roughened by numerous sharp spines and tubercles. The animal which inhabits this shell bores deeply into the rocks of the sea-shore, forming cylindrical holes, in which it lives; and the water which it requires for its food and respiration is conveyed to and from the interior of the shell by a pair of tubes which reach to the outer orifice of its dwelling-place. It is supposed that the Pholas is enabled to bore into the hard rock by means of its large and strong foot, but this is still a matter of dispute.