22 DREISSENIDZ. 
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. Jadiated with green and yellow. 
Var. II. ventricosa. Larger, more solid. 
Var. IIL. complanata. Oval, compressed, beaks 
close to anterior margin. 
Family IIT—DREIssENIDZ. 
Genus.—DREISSENA. 
1. D. potymMéRPHA (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 specimeng are found 
in the Thames. 
