no Descriptive Zoology. 



the muscular border of the mantle is attached to the shell. 

 The edge of the mantle, it was observed, is attached to the 

 edge of the shell. The outermost layer of the shell, the 



Umbo 



FIG. 66. STRUCTURE OF CLAM SHELL. 



Cross section. 



periostracum, is formed by the edge of the mantle, and is 

 horny in composition. Inside this is the prismatic layer, 

 and innermost is the laminated pearly layer. 



Growth of the Clam Shell. The successive concentric 

 lines of growth seen on the outside of the shell mark the 

 growth, each line of growth having once been the ventral 

 edge of the shell. The layers are formed by the mantle, 

 and each new layer is a little wider and longer than the one 

 preceding, and outside of it. The muscles grow and gradu- 

 ally move outward, hence the muscle scar continually widens, 

 forming a triangle. But as the muscles move on, the scar 

 of former years is covered by the new layers formed by the 

 mantle. 



Uses of the Clam Shell. The fresh-water clams are little 

 used as food, but their shells are used largely in making 

 buttons. This is an industry of considerable extent along 

 the Mississippi River and some of its tributaries. 



Respiration in the Clam. The current of water which 

 we saw entering and leaving the clam brings oxygen as well 



