





MOLLUSCA. BIVALVIA. 381 



BIVALVIA. 



ASIPHONIDA. 



Cloak open,' like the aperture of the shell, and permitting the 

 water to come immediately in contact with the mouth and 

 gills. The margin of the cloak has a double fringe of fi- 

 laments. 



I. Valves closed by one adductor muscle. 



A. Shell free, or adhering to other bodies by a byssus only ; 

 furnished with a foot. 

 a. Ligament narrow, and confined to a space under the beaks. 



PECTENIDiE. 



b. Shell compact. 

 Pecten. 

 Lima. 

 Plagiostoma. 

 (Pedum.) 



bb. Shell foliated. 

 Gryphcea. 

 (Vulsella.) 

 (Placuna.) 



aa. Ligament marginal, sublinear, simple or interrupted by the 

 hinge teeth or pits ; shell foliated. Pernad.e. 

 Pema. 

 Gervillea. 

 Crenatula. 

 Inoceramus. 

 (Malleus.) 



AA. Shell fixed or cemented to other bodies ; inequivalve. 



a. Hinge plain or destitute of teeth. Ostreadcs. 

 Ostrea. 

 Dianchora. 

 (Hinnites.) 

 Anomia. 



«a. Hinge furnished with teeth. Spondylitics. 

 Spondylus. 

 Plicatula. 



