56 UNIMUSCULOSA. TRIDACNA. MODIOLA. 



CHAPTER VIII. 



Class. — Conchifera. 

 Second Order. — Unimusculosa, 



<( 



1st family, Tridacnacea. 



Tridacna Ch ama . 



Hippopus Chama, (one species.) 



" In the first genus we find the great Tridacna 

 gigas, the largest and heaviest shell yet known. 

 It sometimes weighs five hundred pounds. The 

 hinge has two teeth, the lunula is open, the 

 valves equal, the ligament exterior. 



" Mytilacea. 



Modiola MyUlus. 



Mytilus Mytilus. 



Pinna Pinna. 



'' The greater part of these genera attach them- 

 selves to marine substances by a byssus. The Mo- 

 diola genus are rarely found fixed. The ligament 

 internal, lodged in a marginal gutter. Beaks 



