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HouD to Know Aiiiericmi Seashells 83
passage of the foot. Some clams, such as Soleii and Eiisis, gape at both ends.
In Ar€a 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 resilmm. In most cases, the two portions
are intimately connected with one another, but in some clams, such as My a
and Mactra, the cartilage is entirely separate (the resilium) and is lodged
within the hinge in a spoon-shaped chojidrophore. 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 inipressions. The interior, concave surface of the valve
Figure 28. \'arious types of bivalve hinges, a, Arcidae {Noetia poiuierosa Say);
b, Spondvlidae {Spondyhis); c, Cardiidae {Dinocardhnn vanhyiiingi Clench and
Smith); d, Veneridae (Tivela stidtonim Mawe); e, Veneridae (Callocardia texas-
iaiia Dall); f, Lucinidae {Phacoides anmdatiis Reeve); g, Mactridae {Mactra alata
Spengler); h, Tellinidae {TelUiia idae Dall); i, Carditidae (Veiiericardia); j, A4ac-
tridae {Ra7igia); k, Crassatellidae (Crassmella himdata Conrad); 1, Periplomatidae
(Periploma discus Stearns); m, Corbulidae (Corbida).