THE PEESHWATEE MUSSEL 77 



I. THE SHELL. 



The shell is a cutioular 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 more pointed, and the dorsal hinge-Hne 

 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 reaUy a median, imperfectly 

 calcified part of the shell itself. It is very narrow 

 in 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 wiU 

 open as soon as these muscles relax. Hence in dead 

 specimens the valves always gape, owing to the un- 

 opposed action of the elastic ligament. 



