140 INTRODUCTION TO ZOOLOGY CHAP. 



FRESH-WATER BIVALVE MOLLUSCS 

 Type : The Swan Mussel (Anodonta Cygnea). 



The Swan Mussel is a large form common in the mud at 

 the bottom of streams. 



The Shell ^ e S ^ e ^ ^^' 8 ^ measures, when full grown, 



from four to six inches in length, two or three 



inches in breadth, and one to two inches through from 



one valve to the other at the thickest part. The sexes are 



FIG. 89. The Swan Mussel (Anodonta 



distinct, and they can be sometimes distinguished by the 

 greater thickness from side to side of the shell of the female. 

 The colour of the shell is a greenish-brown. It is marked 

 with more or less concentric lines of growth, each line repre- 

 senting what was formerly the actual margin of the shell. 

 Its size increases year by year until the mussel is twelve 

 to fourteen years old. 



The lower free margin of each valve of the shell is 

 rounded, but the two valves meet above in an almost straight 

 line, where they are united by an elastic ligament narrow in 

 front but broader behind. This is known as the hinge-line. 

 At the anterior end of the hinge-line is the umbo, the oldest 

 part of each valve. It will be seen that in the swan mussel 

 the shell has grown unequally. round this umbo, hardly any 



