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Rh passage of the foot. Some clams, such as Solen and Ensis, gape at both ends. In Arca there is a small notch or opening on the ventral margin for the passage of the anchoring organ, the byssus. This is called the byssal notch.

The ligament is a brown, horny band located above the hinge, and is generally posterior to the beaks. As a rule, the greater part of the ligament is externally placed on the shell, but in some genera it may be partially or entirely internal. The ligament consists of two distinct parts, which may occur together in the same species or separately in others—the ligament proper and the internal cartilage or resilium. In most cases, the two portions are intimately connected with one another, but in some clams, such as Mya and Mactra, the cartilage is entirely separate (the resilium) and is lodged within the hinge in a spoon-shaped chondrophore. The external ligament is inelastic and insoluble in strong alkali (KOH). The cartilage is very elastic, slightly iridescent and soluble in KOH.

Muscle scars or impressions. The interior, concave surface of the valve

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