_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. [Illustration: FIG. 188.--Hinges of =A=, _Cardita semiorbiculata_ Brug., and =B=, _Unio rectus_ Lam., showing how, in inequilateral shells, the lateral teeth tend to shift their position. _a.m_, anterior adductor, _p.m_, posterior adductor muscle; _c_, _c_, cardinal teeth; _p.l_, posterior lateral teeth; _l_, ligament.] Some bivalves, _e.g._ _Anodonta_, _Ostrea_, _Pedum_, many _Mytilus_, have no hinge teeth at all, in others the laterals are wanting (_Psammobia_, _Diplodonta_). In the Arcadae the hinge consists of a number of very similar denticles, which are often serrated like the teeth of a comb (Fig. 189). [Illustration: FIG. 189.--The hinge in _Arcadae_: =A=, _Nacula Loringi_ Ang. × 4/3; =B=, _Arca granosa_ L.; _u.a_, umbonal area.] [Illustration: FIG. 190.--=A=, _Tridacna scapha_ Lam.; =B=, _Cardium enode_ Sowb., showing the interlocking of the ventral margins.]