The shell is formed of two valves, united by a hinge which is sometimes of the simplest possible description, but which often exhibits a beautiful arrangement of interlocking teeth. A _ligament_ of flexible and elastic substance often holds the two valves together. [Illustration: FIG. 127.--SHELL OF THE PRICKLY COCKLE (_Cardium aculeatum_) SHOWING UMBO AND HINGE; ALSO THE INTERIOR SHOWING THE TEETH] The reader has probably observed that the valves of a dead lamellibranch usually gape. This is due either to the pull exerted by a ligament that is attached to the valves outside the hinge, or to the pressure of an internal cartilage which unites the valves within, and which is compressed when the shell is closed. When the animal is alive, it has the power of closing its shell by the contraction of the adductor muscles, to be presently described, and when the valves are brought together by this means the external ligament is more or less stretched, or the cartilage within, which is also an elastic material, is compressed. Examining the shell from the exterior we observe that each valve has a nucleus (the _umbo_) close to the hinge, round which are usually a number of more or less distinct concentric lines, extending to the lower or ventral margin. This nucleus represents the whole shell of the young mollusc, and the lines are the lines of growth, each one marking the extreme limit of the valve at a particular period of the animal’s existence. Further it will be observed that the lines of growth are often wider apart in some directions than in others, thus denoting the unequal rate of growth that determined the form of the adult shell. [Illustration: FIG. 128.--INTERIOR OF BIVALVE SHELL, SHOWING MUSCULAR SCARS AND PALLIAL LINE] The shell of a bivalve is often made up of two very distinct layers, the outer one called the prismatic layer because, when examined microscopically, it is seen to consist of minute vertical prisms of calcareous matter; and the inner one presenting a beautiful pearly iridescence, due to the fact that it is made up of a number of extremely thin and finely waved layers of calcareous substance that have the power of decomposing light. This latter layer is secreted by the whole surface of the mantle that lies in contact with it, while the outer, prismatic portion of the shell is formed only by the free edge of the mantle; and we often find a distinct line (the _pallial line_), some little distance from the ventral margin that marks the junction of the muscle of the mantle with the shell. The shape of this line is a very important feature of the shell, since it is of great value in the determination of relationships.