72 CONCHOLOGY. 



FAMILY XV. 



Tridacnea. Two genera. 



1. Genus Tridacna. PI. 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 ; branchise 

 long and narrow ; abdominal muscular mass considerable, and 

 giving issue, as if from a cup, to a 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. PI. VIII. 



Animal. As above. 



Shell. More elongated and inequilateral than in the Tridac7ia, 

 the anterior side being longer than the posterior ; the posterior 

 slope closed with a dentated margin. Inhabits the Indian seas. 

 One species. 



Hippopus maculatus. 



