TRIDACNA. 



byssus by which the aniipal fixes itself to the rocks. The 

 posterior dorsal margin is nearly straight, and the liga- 

 ment which is elongated, is affixed to its fulcrum just 

 within the margin, so that it is not wholly external, 

 though it is partly on the outside. The teeth, in one valve, 

 are two, one of which is short and placed close to the 

 umbo, the other elongated and near the margin; in the 

 other valve, three, one of them short, and placed near the 

 umbo, the others more lengthened and posterior. The 

 whole of the ventral margin is deeply sinuated. The mus- 

 cular impression may be traced, a little way within, 

 around the whole margin, and it is more expanded toward 

 the center of the ventral margin. The shells of this 

 Genus are almost always of one colour, most commonly 

 white, sometimes yellow, very seldom orange or rose 

 coloured, but without variegation ; their external surface 

 is covered with broad radiating ribs, which are frequently 

 crossed transversely by broad and high vaulted scales, 

 giving the shells a very handsome appearance. 



This Genus is only found recent and in tropical 

 climates; we believe the East Indian and Australian Seas 

 alone supply specimens of it. It differs from Hippopus 

 in having a large opening placed just before the umbones. 

 With Chama, Lam. it cannot be confounded, for that is an 

 irregular shell affixed by the outside of one valve. 



We have represented Tridacna elongata, JLam, 



