, CONCHOLOGY. 
FAMILY XV. 
Tripacnesa. Two genera. 
1, Genus Tridacna. Pl. VIII. 
Animal. Body somewhat thick; edges inflated, lobes of the 
mantle adhering, and united in nearly all their circumference, so 
as to present but three apertures, the first inferior and anterior, for 
the egress of the foot; the second superior and posterior for the 
bronchial cavity, the third much smaller and in the middle of the 
dorsal edge ; two pairs of labial appendages, thin, nearly filiform, 
in the middle of which is a very small buccal orifice ; branchie 
long and narrow; abdominal muscular mass considerable, and 
giving issue, as if from a cup, toa thick bundle of byssoid muscu- 
lar fibres. 
Shell. Thick, solid, of variable size, regular, triangular, inequi- 
lateral; summits inclined posteriorly, hinge dissimilar, entirely 
anterior to the summit; a lamellous precardinal tooth and two 
distant lateral teeth upon the left valve, corresponding with two 
precardinal lamellous teeth, and one distant lateral tooth upon 
the right valve ; ligament anterior, elongated ; a large, bifid, sub- 
median muscular impression; another anterior one smaller and 
less distinct. Inhabits the Indian Ocean. Seven living species. 
One fossil. 
Tridacna gigas. Tridacna crocea. 
T. squamosa. T. mutica. 
T. elongata. T. serrifera. 
T. pustulosa. 
2. Genus Hippopus. Pl. VIII. 
Animal. As above. 
Shell. More elongated and inequilateral than in the Tridacna, 
the anterior side being longer than the posterior ; the posterior 
slope closed with a dentated margin. Inhabits the Indian seas. 
One species. 
Hippopus maculatus. 
