20 LAND AND FRESH-WATER MOLLUSKS. 
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.—DeEscripTION OF SPECIES. 
Tue fresh-water bivalves of Great Britain are 
comprised in the three families—Mytilide, Union- 
ide, and Cycladide. 
Famity Myritipz (MussEzs). 
This family is typified by the common marine — 
- mussel (Mytilus edulis) of our coasts, and is re- 
presented in our fresh waters by 
Drizssena PpotymMorPHA (the Zebra Mussel) 
(Pl. 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 
