OCR Text |
Show 410 "MOLLUSCA. perior angle, which answers to the head and summit, very ob. tuse. The animal is very singular, inasmuch as it is not, like most of the others, placed in the shell, but is directed, or, as it were, press· ed out before. The anterior side of the mantle is widely opened for the passage of the byssus; a little below the anterior angle is an· other opening which transmits water to the branchic:e, and in the middle of the inferior side is a third and smaller one which cor. responds to the anus, so that the posterior angle transmits nothing, and is only occupied by a cavity of the mantle open at the third orifice, of which we have just spoken. There is but a single transverse muscle, corresponding to the middle of the margin of the valves. In TRIDAONA, Lam., Or the Tridacnc:e properly so called, the front of the shell as well as of the mantle has a wide opening with notched edges for the transmission of the byssus, which latter is evidently tendinous, and continues uninterruptedly with the muscular fibres. Such is the celebrated and enormous shell of India, the Cha· ma gigas, L.; Chemn., VII, xlix, which is decorated with broad ribs relieved by projecting semi-circular scales. Speci· mens have been taken that weighed upwards of three hundred pounds. The tendinous byssus which attaches them to the rocks, is so thick and stout that the axe is required to sever it. The flesh, though tough, is edible. In HrPPOPus, Lam. The shell is closed and flattened before as if truncated(!). In the CHAMA, Brug., Or the true Chamce, the shell is irregular, inequivalve, usually lamellar and rough, adhering to rocks, corals, &c., like that of an Oyster. Its summits are frequently very salient, unequal, and curled up. The internal cavity frequently has the same form with· out any external indication of the fact. The animal,-PsxLoPus, Poli,-has a small foot bent almost like that of man. Its tubes, if it have any, are short and disjointed, and the aperture in the mantle, (1) Chama lazarus, Chemn., VII, li, 507, 509;-Ch. gryphoi'des, lb., 510, 513; -Ch. archineUa, Id. Iii, 52~, 523;-Ch. macrophylla, lb., 514, 515;-Ch. foliacea, lb., 521;-Ch. citrea, Uegenf., IV, 44;-Clt. 'bicornis, lb., 516-520. ACEPIIALA TEST ACEA. 411 which transmits the foot, is not much larger. Some species are found in the Meditel'l'anean. There are also several that are fossil(l). DrcERAs, Lam. Between Diceras and the Chamre there is no essential difference; the cardinal tooth of the former is very thick and the spiral Jines of the valves are sufficiently prominent to remind us of two horns(2). In the IsooARDIA, Lam., We observe a free, regular, and convex shell, with spirally curled summits, divided anteriorly. The animal,-GLossus, Poli,-only differs from that of an ordinary Chama in having a larger and more oval foot, and because the anterior opening of its mantle begins to resume its ordinary proportions. A large, smooth, red species, the CJ,ama cor, L.; Chemn., VII, xlviii, 483, inhabits the Mediterranean(3). FAMILY IV. CARDIACEA. The mantle is open before, and there are, besides, two separate apertures, one for respiration, the other for the freces, which are prolonged in tubes, sometimes distinct, and at others united in one single mass. There is always a transverse muscle at each extremity, and a foot generally used for crawling. It may be considered as a general rule, that those which are furnished with long tubes, live in ooze or in sand. This mode of organization may be recognized on the shell by the more or less depressed contour described by the inser- (1) See the Conchiol. Foss. Subap., of Brocchi, and the Coq. Foss. des Env. de Paris of M. de Lamarck. V (2) Fossil shells from the jurassic strata. Die. arietina, Lam. de Saussure, 0Y· ~ux Alpes, I, pl. ii, f. 1-4. (3) Add Ch. moltkiana, Chemn., VII, xlviii, 484-487. |