CHAPTER VI. 



THE FRESH-WATER MUSSEL. 



LAMELLIBRANCH Molluscs of the Family Unionidce are 

 to be found in nearly all ponds and streams. They are, 

 in spite of their large size, frequently overlooked in con- 

 sequence of their habit of lying almost totally buried in 

 the mud at the bottom of the water. Their presence is 

 usually betrayed by broken shells lying upon the banks 

 or exposing the glittering inner surface beneath the water. 

 These fragments are the relics of the repasts of swans, 

 coots and other water-fowl or occasionally of water-voles. 



The shell of Anodonta cygnea, the Swan Mussel, 

 perhaps the commonest of our Unionidce, is from four 

 to six inches in length, from two to three inches broad, 

 and from one to two and a half thick from right to left. 

 It is of a greenish-brown tint, but is often speckled with 

 white patches in consequence of the surface layer being 

 corroded and the deeper calcareous layer exposed. The 

 two valves are placed on the right and left of the animal's 

 body and resemble each other in shape (equivalve). Each 

 is a reflex (or looking-glass-picture) of the other. The 



