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 byssusj a little below the anterior angle is an- 

 other opening which transmits water to the branchiae, 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 



Tkidacna, Lam., 



Or the Tridacnae 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 



Hippopus, Lam. 

 The shell is closed and flattened before as if truncated(l). In the 



Chama, Brug., 



Or the true Chamae, 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, Psilopus, 

 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.gryphoides, lb., 510, 5l3; 

 Ch. archhiella. Id. lii, 522, 523; CA. macrophylla, lb., 514, 515; CA. foliacea, 

 lb., 521; Ch. citrea, llegenf., IV, 44; Ch, bicornis, lb., 516520. 



