are about eleven or twelve of these chambers, so that although light, the structure is very strong. Upon this "bridge" are placed certain "accessory valves," or plates of shell, to protect the base of the shell; and stretching along the back is a flat plate of the same substance, which guards the opening at the hinge. On the inside of the hinge is seen a curved projecting piece of shell, the use of which seems to be rather obscure. These accessory valves, and the flat piece that protects its back, are seen at fig. 2.
The reader will at once see that there is considerable similarity of structure between the animal of the Piddock and that of the Pelonea, which has just been described, the body being elongated, and the two siphons placed side by side at the extremity. In colour it is whitish, and the foot is nearly as transparent as if made of ice. Fishermen value this creature extremely, as it forms excellent bait, attracting the fish, and adhering firmly to the hook, which is passed through the foot.
The method by which the Piddock bores is now satisfactorily ascertained to be by a peculiar movement of the shell, not unlike that of a brad-awl when a carpenter is boring a hole in wood. The myriad tooth-like projections of the shell act like the teeth of a file, and by constant attrition work the shell into the rock. That they must become clogged by the particles which they cut away from the rock, is evident enough; and in order to free itself from these choking particles, the Piddock constantly deluges the burrow with water, washing out the greater part of the muddy sediment, and enabling the shell to perform its office effectually.
The common Piddock varies greatly in size, the largest being those which bore into soft substances, and the smallest those which inhabit hard limestone. The largest specimen that I have seen is in the possession of Mr. G. B. Sowerby, and is so large as to deserve the name of gigantic. It is found in various
