20 
LAND AND FRESH-WATER HOLLUSKS. 
them for the cabinet, the shells should be first 
cleaned with a brush, and after being immersed 
in boiling water, the animals may be removed. 
The interiors of the shells should be wiped dry, 
and the valves closed, and tied with a moist tape 
or cotton; they should now be allowed to dry 
slowly, for if heated too much they are apt to 
crack. 
II. —Description of Species. 
The fresh-water bivalves of Great Britain are 
comprised in the three families— Mytilidce , Union - 
idee , and Gycladidce. 
Family Mytilid^: (Mussels). 
This family is typified by the common marine 
mussel ( Mytilus edulis) of our coasts, and is re¬ 
presented in our fresh waters by 
Driessena polymorpha (the Zebra Mussel) 
(PI. IV., fig. 25).—This bivalve closely resem¬ 
bles the common mussel; the shell is equi-valve, 
wedge-shaped, rounded behind; the umbones 
are placed at the anterior end; the valves are 
sharply keeled in the middle. The principal 
differences between this genus and Mytilus are 
as follows :—In Mytilus the mantle is open, in 
Driessena closed; in Mytilus the gills adhere 
through their whole length, in Driessena the 
extremities are free. In Driessena the anterior 
