book and, by its physiological activities, secretes the hard valves of calcium carbonate, which thus come to occupy the position of the covers of the book. In the simplest form of mantle the edges are free except on the back, where the hinge of the shell is located, corresponding to the arrangement of a book. Sea water may enter the cavity enclosed by the mantle at almost any place. In many groups of bivalves, however, the mantle edges may be fused, not only along the back where the valves are joined together but along all or most of the lower margins. Openings are usually present to accommodate the foot and siphons when such organs are developed.
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Figure 10. Structure and layers of a clam shell (Tellina tenuis). a, Diagrammatic representation of a small piece of shell; b, Cross-section of shell showing the loose end of periostracum around the margin of the shell. (After Truman 1942.)
From its food supply the clam absorbs minerals into its blood system which are then carried to the mantle. A certain amount of shell deposition takes place along the thickened borders of the mantle, although a small amount, including pearly or nacreous material in some species, is laid down by other parts of this organ. The liquid secretion of lime salts becomes crystallized when mixed in a colloidal albumen which is also produced by the mantle. Several types of shelly material are laid down in definite layers, and the structure and composition may vary depending upon the family or genus of mollusks. The structure of a layer may be prismatic (made up of tiny, individual, closely packed prisms), foliated (layers built up of overlapping leaves), nacreous (mother-of-pearl), granular (like grains of sugar stuck together), crossed lamellar (a common type in which the long lamellae are rectangular), or it may be homogeneous with no visible structure. The mineral character of these layers may be calcite (2.7 times as heavy as water) or aragonite (2.9 times as heavy as water), both of which are forms of calcium carbonate.
The shell of the tellin clam (Tellina), for instance, is made up of three layers of calcium carbonate and the horny periostracum. The latter con-
