They are sometimes called "shell-fish," but this is no good as a distinctive name--since it is applied in the fish-trade to lobsters, crabs, and shrimps as well as to Molluscs. Lobsters, crabs, and shrimps are Crustacea, and totally different in their architecture and their mechanism from Molluscs. Familiar examples of Molluscs are the oyster, the mussel, the various "clams," and, again, the snails, periwinkles, whelks, and limpets. It is the shells of these animals which are "true" shells in the sense in which the word is used by "collectors" of shells, and in the sense in which we speak of "the shells of the seashore." These shells are usually very hard, solid things, made up of layers of lime-salts and horny matter mixed, and they remain for a long time undestroyed, washed about by the currents of the sea, and thrown up on to the beach, after the soft, oozy creature which formed them--chemically secreted them on its soft skin--has decomposed and disappeared. They are readily distinguished into two sorts--(1) those which are formed in pairs, or "bivalves," each member of the pair being called a "valve"; and (2) those which are single, or "univalves," often spirally twisted, as are those of snails and whelks, but sometimes cap-like or basin-like, as are the shells of the limpets. There is not so great a difference between bivalve and univalve shells as there seems to be at first sight. For if you examine the pair of shells of a mussel or a clam when they are quite fresh, you will find that the valves are joined together by a horny, elastic substance, and are, in fact, only one horny shell, or covering, which is made hard by lime deposited on the right and on the left, as two plates or valves, but is left soft and uncalcified along a line where these two valves meet, so as to allow them to move and gape, as it were, on an elastic hinge. It is the fact that the two valves of the shell of the bivalve, lying right and left on its body, correspond to the single shell of the snail or limpet, which differs from the bivalve-shell in not being divided along the back by a soft part into right and left pieces. That there is this real agreement between bivalve and univalve molluscs is quite evident when we examine the soft animal which forms the shell and is protected by it. [Illustration: FIG. 19.--Some British Marine Bivalve Molluscs. a, The smaller Piddock, Pholas parva, which bores into chalk, clay, and hard rock. Natural size. b, The Razor-shell, Solen siliqua. The muscular foot is seen protruding from the shell. One-third the natural size, linear. c, Venus verrucosa. Natural size. d, Cardium echinatum. Two-thirds the natural size, linear. e, Pinna pectinata, the "cappy longy." One-fifth of the natural size, linear.]