1. Tridacna. The Clam Shell. Six species.
The most ponderous shell known, sometimes measuring several feet in length, and sometimes weighing five hundred pounds.
By Linnæus this genus was classed with the Chama, but the characteristic distinctions are so great that they are easily known. The Chama is irregular, has but one tooth, and is fastened to other substances by the lower valve; the Tridacna is equivalve, has but two teeth, and is affixed to other bodies by a byssus consisting of filiform tendons.
The animal inhabiting this shell is said to produce very fine pearls, but there is no pearly appearance on the valves.
Shell thick, solid, varying in size; some are very small and some very large; regular, triangular, more or less inequilateral; the summits inclined backward; hinge dissimilar, entirely anterior to the summits; one lamellous precardinal tooth, and two distant lateral teeth on the left valve, corresponding to two lamellous precardinal teeth and one remote lateral tooth in the right valve; ligament anterior, elongated; one forked sub-median muscular impression, almost marginal and oftentimes nearly obsolete; valves with broad, rounded longitudinal ribs, armed with vaulted scales, more or less elevated; posterior slope heart-shaped, and widely gaping.
Tridacna gigas.
T. elongata.
T. squamosa.
T. crocea.
T. mutica.