[AS.] The outer part of an egg or nut, the pod of peas, and the hard covering of some kinds of ocean animals, Shell-fish are usually uni valves, having one part;' or bivalves, having two parts joined with a hinge. Shells are useful for protection, and their strength and thickness are generally in proportion to the dangers to which the animal is exposed. Those in-habitingshallow places near the shore and exposed to the beating of the waves have stronger shells than those living in deep water. Fresh-water molluscs generally havedelicate shells. The thin layers of the oyster-shell are deposits of shelly matter showing the lines of growth. All living shells have an outer layer of animal matter called epidermis, and they have no lustre till this is taken off. Mussel-shells show beautiful blue tints when the epidermis is removed. The bodies of all shell-fishes are enclosed by a delicate membrane called a mantle, which secretes the shell. Among other animals the tortoise has a shell,whose upper part is made of the flattened spines of the vertebrae, and of the ribs, the shelly plates being merely portions of the skin hardened into shell. (See Tortoise.) There are many other animals that form shells, some of them being microscopic. Chalk is largely made up of the shells of these.

SPINY SHELL.

SPINY SHELL.