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.
T. serrifera.
_T. gigas._ The giant Tridacna. Pl. 12, fig. 3.
Species of which the shell is sometimes very large, white, transversely
ovate or elongated; the anterior side longer than the posterior; broad
ribs covered with vaulted scales; when of a pink or orange colour,
greatly valued.