Distinguished from the Chama by the want of teeth in the hinge, and being of a pearly substance. It is a rare shell, only found in deep water, where it is attached to the rocks by the lower valve. Its irregular form is perhaps occasioned by the lower valve adapting itself to the form of the body to which it is affixed. Two species are fluviatile and two marine. Shell adhering, irregular, thick, pearly, inequivalve, inequilateral; summits thick, little evident; hinge toothless, callous, thick, irregular; longitudinal, sub-dorsal ligament, partly exterior and partly penetrating into the shell; two oblong muscular impressions, one inferior and anterior, the other superior and sub-posterior; with or without an oblong incrusted callosity on the base of the valve. Etheria elliptica. E. semilunata. E. trigonula. E. transversa. _E. elliptica._ The oval Etheria. Pl. 11, fig. 1. Species with an oblong callosity on the anterior part of the shell; oval, depressed, dilated towards the umbones; summits distant. _E. semilunata._ The semilunar Etheria. Species without callosity at the base. FAMILY XV. TRIDACNITES. Two genera. 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. T. serrifera. _T. gigas._ The giant Tridacna. Pl. 12, fig. 3.