10 LAND AND FRESH-WATER MOLLUSKS. 
CHAPTER II. 
BIVALVES (Conchifera). 
 [.—Srructure oF THE Parnters’ Mussen 
(Unio pictorum). 
\HE most common of the fresh-water bivalves 
is the swan mussel, an ordinary tenant 
of our rivers, streams, and ponds, lying half- 
buried in the mud. We select, however, the 
less widely distributed Painters’ Mussel, as a 
type of the bivalves for study; because the 
characters of the class are better exhibited by it 
than by the swan mussel. 
Let us first glance at the characters afforded 
by the shelly covering. The shellis at once seen 
to be composed of two pieces or valves; one is 
applied to the left side of the body of the mussel, 
and the other to the right: the valves are equal, and 
the shell is therefore said to be equivalve. The 
line along which the two valves are joined is the 
hinge, and that part in its vicinity, because it 
covers the back of the animal, is called the 
dorsal region; that opposite to it, the ventral 
region. The rounded margin is the anterior 
