_Shell._ Irregular, adhering, inequivalve, inequilateral; summits more or less twisted spirally, especially in the lower valve by which these animals have the faculty of affixing themselves to other bodies; hinge dissimilar, thick, formed by a single lamellous tooth arcuated, subcrenulated, postcardinal and articulated in a furrow of the same form; ligament anterior and postapicial; two large muscular impressions, distant. British and American seas. Eighteen living species. Chama damæcornis. C. crenulata. C. arcinella. C. cristella. C. Lazarus. C. gryphoides. C. unicornis. C. radians. C. æruginosa. C. decussata. C. ruderalis. C. Japonica. C. limbula. C. asperella. C. albida. C. croceata. · · · · · C. exogyra.* C. Florida.* 3. Genus _Etheria_. Pl. VIII. _Animal._ Unknown. _Shell._ Adhering, irregular, thick, much nacred, inequilateral, inequivalve; summits subcephalic, thick, indistinct, in a species of heel, growing longer with age; hinge without teeth, callous, irregular and thick; ligament subdorsal in the exterior portion of the shell, and prolonged to a point internally; two irregular oblong muscular impressions, one superior and subposterior, the other inferior and anterior, with a marginal palleal impression. Inhabits the Indian Ocean. Five species. Etheria elliptica. E. trigonula. E. semilunata. E. transversa. E. Carteronii. FAMILY XV. TRIDACNEA. Two genera. 1. Genus _Tridacna_. Pl. VIII. _Animal._ Body somewhat thick; edges inflated, lobes of the mantle adhering, and united in nearly all their circumference, so as to present but three apertures, the first inferior and anterior, for the egress of the foot; the second superior and posterior for the bronchial cavity, the third much smaller and in the middle of the dorsal edge; two pairs of labial appendages, thin, nearly filiform, in the middle of which is a very small buccal orifice; branchiæ long and narrow; abdominal muscular mass considerable, and giving issue, as if from a cup, to a thick bundle of byssoid muscular fibres. _Shell._ Thick, solid, of variable size, regular, triangular, inequilateral; summits inclined posteriorly, hinge dissimilar, entirely anterior to the summit; a lamellous precardinal tooth and two distant lateral teeth upon the left valve, corresponding with two precardinal lamellous teeth, and one distant lateral tooth upon the right valve; ligament anterior, elongated; a large, bifid, submedian muscular impression; another anterior one smaller and less distinct. Inhabits the Indian Ocean. Seven living species. One fossil. Tridacna gigas. T. squamosa. T. elongata. T. crocea. T. mutica. T. serrifera. T. pustulosa. 2. Genus _Hippopus_. Pl. VIII. _Animal._ As above.