PARTS OP THE SHELL. 
11 
like the oyster, have hut a single adductor, which 
leaves its scar near the center of the shell. Most 
of the Lamellibranchs, however, have two mus- 
cles, one near each end of the shell. These mus- 
cles, when fresh, are white and glistening, and 
their strength is very considerable. They are, 
in fact, the lock by which the poor mollusk fast- 
ens the door of his fortress against all intruders, 
and so firm is it that it generally prevails, until 
some strong-jawed fish crushes the shell, or some 
insidious borer drills a hole through its hard 
plates. These muscles leave markings on the 
shells, which are important guides in the deter 
ruination of the species. 
The mantle , or skin of the mollusk, is partial- 
ly grown to the shell, and leaves a distinct mark 
called the pallictl line, at the place where it be- 
comes free. This line sometimes makes a deep 
bend inward, called the pallial simis , which is 
occupied by the breathing siphons when they are 
withdrawn into the shell. The part of the shell 
occupied by these siphons is called the posterior, 
and the other extremity the anterior portion. 
When the shell is placed upon its edge, with the 
posterior portion toward you, the right and left 
valves correspond to your right and left hands. 
In external sculpture, those lines which run 
from the umbo to the edge of the valve are call- 
ed radial lines , and those which mark the size of 
the shell in its progressive stages of growth are 
concentric lines . The peculiar heart-shaped de- 
pression found on some shells, beneath and in 
