90 THE FRESHWATER MUSSEL. 



I. THE SHELL 



The shell is a cuticular product formed by the underlying 

 epidermis. It is, therefore, entirely outside the body, and 

 incapable of growth except by addition of new rings or new 

 layers to the part already formed. It consists of an organic 

 basis impregnated with calcareous salts. 



A. The Outer Surface of the Shell. 



1. Shape. The anterior end of the shell is rounded, the 



posterior end more pointed, and the dorsal hinge- 

 line straight. 



2. The umbo is the oldest part of each valve. It is close 



to the hinge-line, and about a quarter of the length 

 of the shell from its anterior end. 



3. The lines of growth are the concentric curves on the 



surface of the shell, marking successive stages in 

 its growth. Each of these lines was at one time the 

 margin of the shell. 



The shell does not grow uniformly in all direc- 

 tions. Growth is much more rapid towards the 

 ventral than towards the dorsal surface, and more 

 rapid posteriorly than anteriorly ; hence the umbo 

 or oldest part is close to the dorsal edge, and nearer 

 the anterior than the posterior end. 



4. The ligament connects the two valves together along 



the mid-dorsal line : it is a median, imperfectly 

 calcined, and elastic part of the shell itself. It is 

 very narrow at the anterior part of the hinge-line, 

 and much broader behind. As the valves are only 

 kept closed by contraction of the adductor muscles, 

 they will open as soon as these muscles relax. 

 Hence in dead specimens the valves always gape 

 owing to the unopposed action of the elastic liga- 

 ment. 



B. The Inner Surface of the Shell. 



Wedge open the valves of the shell with the handle of a 



