76 THE FRESHWATER MUSSEL 



. 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. 



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 ligament. 



A. The Outer Surface of the Shell. 



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



posterior more pointed, and the dorsal hinge-line 

 straight. In the female the shell is more convex 

 than in the male. 



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. It has usually a 

 pearly white appearance, owing to the outer layer of 

 the shell having been rubbed off. 



3. The lines of growth are the series of concentric rings on 



the surface of the shell, marking successive stages in 

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

 actual margin of the shell. 



The shell does not grow uniformly in all direc- 

 tions. Growth is much more rapid towards the 

 ventral than 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 really a median, imperfectly 

 calcined part of the shell itself. It is very narrow 

 in the anterior part of the hinge -line, and much 

 broader behind. 



