The ligament is a more or less elastic band, which unites the two valves along a line adjacent to the umbones. As a rule, the greater part of the ligament is external to the shell, but it may be entirely internal. It is placed, normally, behind the umbones, but in a few cases, when the hinge line is very long (Arca, Pectunculus), it extends in front of them as well. The edges of the valves, when the ligament is mainly external, are more or less excavated for its reception. When internal it is generally contained in a groove or spoon-shaped pit, known as the fossette (compare Fig. [187]).
Fig. 186.—Venus subrostrata Lam.: es, escutcheon; li, ligament; lu, lunule; u, u, umbones.
The ligament consists of two distinct parts, which may occur together or separately, the external, or ligament proper, and the internal, or cartilage. Only the external portion can be seen when the valves are closed. As a rule, the two portions are intimately connected with one another, the ligament folding over the cartilage, but in some cases, e.g. Mya, Mactra, where the cartilage is lodged within the hinge, they are completely disconnected (Fig. [187]).
In Pecten the external ligament is very thin, and runs along the dorsal margin, while the internal ligament is large, solid, and situated in a shallow pit. In Perna, where the hinge is toothless, the ligament is folded into a number of transverse ridges, which fit into corresponding grooves in the shell.
The ligament proper is inelastic and insoluble in caustic potash. The cartilage is very elastic, composed of parallel fibres, slightly iridescent, and soluble in caustic potash.
The operation of the ligament—using the word as including the whole ligamental process—is in opposition to that of the adductor muscles. When the latter close the valves, they compress the ligament, an action which its elasticity resists: thus its operation tends in part towards keeping the valves open. But when ligament and cartilage are both fully developed, they work in opposition to one another, the ligament, by its resistance to compression, preventing any straining of the adductor muscles when the valves are open, and the cartilage, for the same reason, preventing the ventral margins of the shell from closing too rapidly upon one another when the valves are being shut.
Fig. 187.—Hinge of A, right valve, and B, left valve of Mulinia edulis King; ca, cardinals; l.a, anterior laterals; l.p, posterior laterals; f, fossette; c, cartilage; l, ligament.
The Hinge.—The valves of Pelecypoda are generally articulated, below the umbones, by a hinge which is furnished, in the majority of cases, with interlocking teeth, small pits or depressions in each valve corresponding to the teeth in the other. The teeth are distinguished as cardinal, or those immediately below the umbo, and lateral, or those to either side of the cardinals, the latter being also distinguished as anterior and posterior laterals, according as they are before or behind the umbo (Fig. [184]). In shells which are tolerably equilateral there is no difficulty in distinguishing between cardinal and lateral teeth. But when they are very inequilateral, the whole hinge may share in the inequality of growth, and an anterior lateral may be thrown backward and simulate a cardinal, or a cardinal may be thrown backward and simulate a posterior lateral (e.g. Cardita, Unio, Fig. [188]). In many Chama the cardinals are pushed up into the umbo and become a mere ridge, while the strong anterior lateral becomes nearly central and simulates a cardinal.