ACEPHALA TESTACEA. 279 



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 branchix. 



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

 the margin of the valves. In Tridacna, 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 evi- 

 dently tendinous, andcontinues uninterruptedly with the muscular fibres. 



Such is the celebrated and enormous shell of India, the Chama gigas, L. ; 

 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 mus- 

 cle 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 in the shell by the more or less depressed con- 

 tour described by the insertion of the edges of the mantle previous to' 

 its uniting with the impression of the posterior transverse muscle. 



Cardium, Lin. 

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

 with salient summits curved towards the hinge, which, when viewing it side- 

 wise, gives it the figure of a hearty 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 elbowin the middle and with its point 

 directed forwards, and two short or but moderately long tubes. 



DoNAX, Lin. 



The JDonaces 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 



