CONCHOLOGY. 71 



Other superior and sub-posterior; with or without an oblong 

 incrusted callosity on the base of the valve. 



Elheria elliptica. Etheria trigonula. 



E. semilunata, . E. transversa. 



E. elliptica. The oval Etheria. PI. 11, fig. 1. 



Species with an oblong callosity on the anterior part of 

 the siiell ; oval, depressed, dilated towards the umbones ; 

 summits distant. 



E. semilunata. The semilunar Elheria. 

 Species without callosity at the base. 



FAMILY XV. 



Tridacnites. Two genera. 

 1. Tridacna. The Clamp Shell. Six species. 



The most ponderous shell known, sometimes measuring 

 several feet in length, and sometimes weighing five hundred 

 pounds. 



By Linnaeus this genus was classed with the Chama, but 

 the characteristic distinctions are so great that they are easily 

 known. The Chama is irregular, has but one tooth, and is 

 fastened to other substances by the lower valve ; the Tri- 

 dacna is equivalve, has but two teeth, and is aflfixed to other 

 bodies by a byssus consisting of filiform tendons. 



The animal inhabiting this shell is said to produce very 

 fine pearls, but there is no pearly appearance on the valves. 



Shell thick, solid, varying in size ; some are very small 

 and some very large ; regular, triangular, more or less ine- 

 quilateral; the summits inclined backward; hinge dissimi- 

 lar, entirely anterior to the summits ; one lamellous precar- 

 dinal tooth, and two distant lateral teeth on the left valve, 

 corresponding to two lamellous precardinal teeth and one 

 remote lateral tooth in the right valve ; ligament anterior, 

 elongated ; one forked sub-median muscular impression, al- 

 most marginal and oftentimes nearly obsolete ; valves with 

 broad, rounded longitudinal ribs, armed with vaulted scales, 



