976 



FRESH-WATER BIOLOGY 



ASSIMENIIDAE 



FlG. XI. Assimenia grayana Leach. 



AMNICOLIDAE 



FIG. XII. Amnicola, porata Say. 



PLEUROCERATIDAE 



FIG. XIII. Anculosa dissimilis Say. 



NERITIDAE 



FIG. XIV. Neritina reclivata Say. 



The classification of the Lamellibranchia is based primarily on 

 the structure of the gill. Each gill consists of "a hollow vascular 

 axis bearing on each face a row of more or less flattened filaments 

 which are nothing more than simple expansions of the axis." The 

 Lamellibranchia are divided into four orders, according as these 

 filaments are flat and non-reflected (Protobranchia) or parallel, 

 ventrally directed and reflected (Filibranchia), or are united at 



