ORDER TESTACEA. 109 
to say, the left valve near the summit is provided with a tooth, 
and further back with a salient plate, which are received into 
corresponding fosse of the right valve. This genus has 
necessarily been divided into the 
TRIDACNA, Brug. 
The shell greatly elongated transversely, and equivalve ; the 
superior angle, which answers to the head and summit, very 
obtuse. 
The animal is very singular, inasmuch as it is not, like many 
others of the kind, placed in the shell, but its parts are directed, 
or, as it were, pressed out towards the front. 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 gills; and in the middle of the 
inferior side is a third and smaller one, which corresponds to 
the anus, so that the posterior angle transmits nothing, and is 
merely occupied by a cavity of the mantle, open only 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 
TRIDACNA, Lam., 
Or the tridacna 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 is uninterruptedly continuous with the muscu- 
lar fibres. 
Such is the celebrated and enormous shell of India, the 
Chama 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 that attaches them to the 
