Shell irregular, adherent, inequivalve, inequilateral; summits more or less twisted spirally, especially in the lower valve; some from left to right, others from right to left; hinge dissimilar, large, formed by one lamellous, arched, sub-crenulated, post-cardinal tooth, articulating into a furrow of the same form; exterior, post-apicial, slightly inserted; two large and rather distant muscular impressions.
Chama Lazarus. C. damæcornis. C. gryphoides. C. crenulata. C. unicornis, C. arcinella. C. radians. C. cristella. C. florida. C. limbula. C. æruginosa. C. asperella. C. decussata. C. albida. C. ruderalis. C. croceata. C. Japonica.
_C. Lazarus._ Lazarus’s Chama. Pl. 12, fig. 2.
Species of which the summits twist from left to right; imbricated, dilated, waved foliations; striated obsoletely; exterior white, orange, red, or yellow; interior white.
Species of which the summits twist from right to left; the pink-coloured ones of this species are the most prized.
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.
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.