BIVALVE SHELLS or BIVALVES are those testaceous coverings of mollusks which consist of two concave plates or takes, united by .a hinge. So long as molluscons animals provided with shells were considered by naturalists alinost exclusively with respect to these, the order of B. S., originally established by Aristotle, retained its place (see CON UROLOGY); and indeed the external character upon which it is founded is closely con nected with sonic of the important structural characters according to which mollusks are now classified. See NoLLusoA. A vast majority of recent B. S. belong to Cuvier's testaeeints order of acephalous raollusca, the lamellibranchiate (q.v.) /not/I/sea of Owen, although with them are classed sonic which were placed among multivalres (q.v.) by conchologists on account of accessory valves which they possess, and some which have a calcareous tube superadded to the true valves, or even takiiig their place as the chief covering of the animal. There are also mollusks of the class brachiopods (q.v.), palliobranehiata, which possess B. S., as the terebratuler, or lamp-shells (q.v.), etc. The structure of the shell, however, when closely examined. is found to be different in these two classes (see Sittn.t.), although its general appearance is much the stone. A very large proportion of the B. S. of the older fossiliferous rocks belong to the clasa brachlopoda.
In the brachiopoda, one valve is ventral, and the other dorsal; in the lamdlibranehi ata, the one is applied to the right side, and the other to the left side of the animal. The valves of ordinary II. S consist of layers, of which the outermost is always the smallest; and each inner one extends a little beyond it, so that the shell becomes thicker and stronger as it increases in length and breadth. The valves are connected at the
hinge by an elastic ligament; and in general this consists of two parts, more or less dis tinct—one on the outside, to which the name ligament is sometimes restricted, and which is stretched by the closing of the valves; another, sometimes called the spring, more internal, which is compressed by the closing of the valves. and tends to open them when the compressing force of the adductor muscle or muscles is removed, the effect of which is to be seen in the Of the shell when the animal is dead. The hinge is often furnished with teeth which lock into each other; sometimes it is quite destitute of them; sometimes the hinge-line is curved, sometimes straight. Concbological classification has been much founded upon characters taken from this part. The valves of some B. S. are equal and symmetrical, in others they are different from one another, particularly In those mollusks which, like the oyster, attach themselves permanently by one valve to some fixed substance,' as a rock. the valves of B. S. close completely at the pleas ure of the animal, those of others always gape somewhere.
The point at the hinge, from which the formation of cach.valve has proceeded, is Called the umbra. On the side of the umbo opposite to the ligament there is usually a small depression called the lunule. The marks, familiar to every one, upon the inside of a bivalve shell, are the impressions of the mantle of the (lamellibranchiate) mollusk, and of the adductor muscle or muscles.