22 DREISSENID^E. 



Found in the same localities as A. cygnea. This 

 species may be distinguished from the last by its smaller 

 size and the angle its hinge line makes with the lower 

 margin. 



Var. I. radiata. Radiated with green and yellow. 



Var. II. ventricosa. Larger, more solid. 



Var. III. complanata. Oval, compressed, beaks 

 close to anterior margin. 



Family III. DREISSENID^E. 

 Genus. DREISSENA. 



1. D. POLYMORPHA (many -shaped). 



Shell triangular, sharply keeled, lower margin curved 

 inwards, solid; ligament long and narrow. Animal fur- 

 nished with a byssus. 



There is no mistaking this shell. Its triangular 

 carinated shape is sufficient to identify it at once. 



It inhabits rivers, lakes, and canals in several parts 

 of England. I have found it attached to iron pipes in 

 a reservoir near Manchester. Fine specimens are found 

 in the Thames. 



