Pelecypoda. 



103 



of clams are distinguished by the size and shape of the 

 shell, the relative proportions of the parts, the color mark- 

 ings, smoothness or roughness of the shell, the thickness 

 of the shell, and by the hinge teeth on the interior. 



Umbo 



Dorsal margin 

 Hinge ligament 



Line of growth 

 Ventral margin 



FIG. 62. EXTERNAL FEATURES OF A CLAM SHELL. 



Outside of left valve. 



The Inside of the Clam Shell. After the soft parts are 

 removed, the following internal markings are usually plainly 

 seen. The scars are the places of attachment of the mus- 

 cles, of which the most notable are the anterior and poste- 

 rior adductor muscle scars. Just below and posterior to the 

 anterior adductor scar is the scar of the protractor of the 

 foot. Above the anterior adductor scar is the scar of 

 the anterior retractor of the foot. Just above and anterior 

 to the posterior adductor scar is the scar of the poste- 

 rior retractor of the foot. There are also a few small scars 

 of muscles in the dorsal regions. In removing the body of 



