42 GENERA OF SHELLS. 



which it is suspended to the rocks, however large 

 and heavy it may be. The T. gigas is the largest 

 shell known. The largest known is said to weigh five 

 hundred pounds. 



T. gigas, elongata, squamosa, crocea, mutica, serrifera, 

 pustulosa. 



HIPPOPUS. Shell equivalve, regular, ine- 

 quilateral, transverse, closed at the lunule. 

 Hinge with two compressed, unequal, anterior 

 and entering teeth ; ligament marginal, ex- 

 terior. 



The hippopus differs from tridacna by having the 

 lunule shut ; wherefore it cannot fix itself to the rocks 

 by a byssus, and consequently must have a different 

 organization from that of the preceding genus. 



H. maculatus. 



MYTILACEA. 



Ligament at the hinge subinternal, marginal, linear, very 

 entire, occupying a great part of the anterior margin; 

 rarely foliated. 



MODIOLA. Shell subtransverse, equivalve, 

 regular, posterior side very short ; beaks nearly 

 lateral, depressed on the short side. Hinge 

 toothless, lateral, linear ; ligament cardinal, 

 mostly interior, received into a marginal 

 groove ; muscular impression sublateral, elon- 

 gated, securiform. 



The modiolse differ from the mytili in being rather 

 transverse than longitudinal shells, the beaks not being 

 truly terminal, the posterior side extending a little be- 

 yond them ; moreover, they are rarely fixed by a 



