other and to determine which is the anterior or posterior end, for many identification features are used in relation to these orientations. The dorsal or upper margin is located on the beak or hinge side; the ventral margin is the opposite side. The beaks usually are pointed or curved toward the anterior end which is generally the less pointed end of the shell. The ligament in the great majority of cases is posterior to the beaks. When present, the heart-shaped impression called the lunule is anterior to the beaks. When a clam is placed on its ventral margins on the table with the dorsal hinge margin up, and with the anterior end away from the observer, the right valve is on the right, the left valve to the left. Another quick way is to observe the concave, interior of a valve with the hinge margin away from the observer and to locate the U-shaped pallial sinus impression (see below). If the sinus opens toward the left, it is a left valve, and vice versa for the right valve.
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Figure 27. Parts of the bivalve shell.
In most bivalves, the two valves are of the same size (equivalve), but in some genera one valve is larger and slightly overlaps the other (inequivalve). In Ostrea, Pandora and Lyonsia, the left valve is the larger; in Corbula, the right valve is the larger. A bivalve is said to be equilateral when the beak is midway between the anterior and posterior ends of the valve. Most bivalves, however, are inequilateral with the beak placed nearer one end.
In many forms, the margins of the valves do not fit closely together, but have an opening called the gape somewhere along the margin. In the Softshelled Clam, Mya, the gape is posterior and through it protrudes the siphon (siphonal gape); in Rocellaria it is anterior and large and serves for the
