PARTS OF THE SHELL. 11 



like the oyster, have but a single adductor, which 

 leaves its scar near the center of the shell. Most 

 of the LamcUibranchs, 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- 

 \j grown to the shell, and leaves a distinct mark 

 called the pallial line, at the place where it be- 

 comes free. This line sometimes makes a deep 

 bend inward, called the pallial shins, 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 withdrawn into the shell.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 yon, 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 



