158 



PARTS OF THE SHELL. 



a little triangle; the long, slender one to the left, is 

 the lateral tooth, and is marked /. /. The point of 

 the shell, which is usually curved inward, is called 

 the beak, or umbo (plural umbones), and is marked 

 u, in the cut. The two valves are drawn together 

 by two strong adductor muscles which run from one 

 to the other, and which are firmly grown to the shells. 

 The scars, showing where the muscles have been 



Fig. 136. 



removed, are shown in the cut at 



and a\ the former 



being called the anterior and the latter the posterior 

 muscle-scar. 



While these muscles can easily hold the two valves 

 together very firmly, there must be some provision 

 for opening them again. Muscles would not answer, 

 for muscles can pull but cannot push; so a strong 

 spring is provided to act in opposition to the muscles. 



