266 MOLLUSCA. 



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, pressed out before. The 

 anterior side of the mantle is widely opened for the passage of the byssus ; a 

 little below the anterior angle is another opening, which transmits water to the 

 branchic. 



There is but a single transverse muscle, corresponding to the middle of the 

 margin of the valves. In TRIDACNA, Lam., or the Tridacnse 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 Chama gigas, Lin. ; 

 which is decorated with broad ribs relieved by projecting semi-circular scales. 

 Specimens 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. 



FAMILY IV. 



CARDIACEA. 



THE mantle is open before, and there are, besides, two separate apertures, 

 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 organisation may be recognised in the shell by the more or less depressed 

 contour described by the insertion of the edges of the mantle previous to its 

 uniting with the impression of the posterior transverse muscle. 



CARDIUM, Linnceus. 



The Cardia, like many other bivalves, have an equivalve, convex shell, with 

 salient summits curved towards the hinge, which, when viewing it sidewise, 

 gives it the figure of a heart ; hence its name of Cardium, Heart, &c. The 

 animal, CERASTES, Poli, has generally an ample aperture in the mantle, a 

 very large foot forming an elbow in the middle, and with its point directed 

 forwards, and two short or but moderately long tubes. 



DONAX, Linnaeus. 



The Donaces have nearly the same kind of hinge as the Cardia, but their 

 shell is of a very different form, being a triangle, of which the obtuse angle 

 is at the summit of the valves, and the base at their edge, and of which the 

 shortest side is that of the ligament, or the posterior side, a rare circumstance 

 in this degree, among bivalves. They are generally small, and prettily striated 

 from the summits to the edges ; their animal, PERON^BA, Poli, is furnished with 

 long tubes, which are received into a sinus of the mantle. The 



