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84 possesses a number of useful identification features. The large muscles which serve to close the valves leave round impressions on the surface. When two muscles are present, as in the venus, lucine, tellin and other clams, they are known as the anterior and posterior muscle scars respectively. The fine, single-lined impression produced by the muscular edge of the mantle is known as the pallial line. The pallial line may have a U-shaped notch at the posterior end of the valve indicating the presence of a siphon and its siphonal muscles. This is known as the pallial sinus. It is entirely absent in genera possessing no retractile siphons.

The hinge. This is one of the most important identification features in the bivalves, and often many hours of fruitless search can be avoided when the major types of hinges and their various parts are understood. There are many types of hinges from those without teeth (edentulous) to those with a complex pattern. We have figured below some of the major types of hinges. The teeth are distinguished as cardinals, or those immediately below the umbo, and the laterals, or those on either side of the cardinals. In many inequilateral bivalves the teeth have become so distorted or set out of place that it is often difficult to distinguish the cardinals from the laterals or to determine which ones are absent. We have labeled the teeth in several groups in the systematic section of this book to overcome this difficulty. In Chama, for instance, the cardinals have been pushed up into the umbo and have become a mere ridge, while the strong anterior lateral has become nearly central and simulates a cardinal.

Sculpture. In many groups, such as the scallops (Pecten), sculpture is of paramount importance in determining species. In most other groups it is used in conjunction with other characters. There are two major types of sculpture—concentric and radial—and both of these may be present in many forms, such as ridges, ribs, nodules, spines, foliaceous processes (leaf-like), threads, beads, indented striae (fine lines), etc. Concentric growth lines of varying degree of development are seen on most bivalves. They are always parallel to the margins of the valves, may be exceedingly fine or very coarse, and they generally indicate former growth and resting stages. Radial sculpture, running from the umbones to the lower or end margins of the valves, is exemplified in the ribs of Cardium (pl. 32), Pecten (pl. 33) and others. Concentric and radial sculpture may occur together to form a cancellate sculpture as in Chione cancellata (pl. 39h). In a few genera, such as Poromya, the valve’s surface may be granulose, as if finely sugar-coated.

The periostracum or protective chitinous sheath overlaying the exterior of the valves is present in most bivalves. It may be extremely thin and transparent so that it imparts a high gloss to the shell, or it may be thick and matted or even very coarse and stringy so that the valves appear to be bearded, as in Volsella and Arca.