CLASSIFICATION 31 
to those in which the umbones are straight, ‘“ ortho- 
gyre,” or are directed backwards, “ opisthogyre.” 
The last-named condition may be the pelecypod 
equivalent of hypertrophy in the Gastropod shell. 
More often than not the two valves are of equal size 
and shape, and the shell is “equivalve,” as in the 
Cockle ; sometimes, however, as in the Oyster, one 
valve is smaller than the other, and the shell is 
“inequivalve.” The Bivalves are all more or less 
“inequilateral”’—that is to say, if a line be drawn 
from the umbo to the ventral margin of the valve, 
the portion on one side of the line, usually the front 
one, will be found to be smaller than that on the 
other. When the shell is shut it is said to be 
“close” if the valves fit accurately, and to “gape” 
if openings be left (Plate XVII., Fig. 5). 
Near the umbones the two valves are united by a 
chitinous “ligament ”’ (Plate XVI., Fig. 2, /), formerly 
known under the misleading name of “ cartilage,” 
which is made up of an outer non-elastic layer and 
an inner, fibrous, elastic layer. The whole forms a 
sort of C spring which tends to open the valves, the 
act of closing being effected by two powerful 
“adductor” muscles that pass from one valve to 
the other, and are situated at either end of the axis 
of the body. 
To prevent the opposed valves from shifting when 
closed, a series of projections, or “teeth,” fitting 
into each other, are developed near the ligament. 
