62 Our British Snails 



and black, beaks always eroded. Mainly found 

 in mountain streams. Its pearls are few and 

 poor compared with those of marine shells ; but 

 they attracted the notice of Caesar and so hastened 

 the conquest (and development) of Britain. 



Anodonta cygnea.- — (In this genus the hinge 

 is toothless, whence its generic name. The specific 

 names cygnea and anatina medin "swan" and 

 "duck," in reference to their comparative size). 

 This is the largest of our freshwater shells, reach- 

 ing even 9 inches in breadth by 4 J in length. 

 Common in ponds and slow water. Sometimes 

 the shells are yellowish green with rays of the 

 same colour. 



Anodonta anatina. — Doubtful if this is a sepa- 

 rate species or only a smaller form. The hinge 

 line is raised instead of being straight, and the 

 posterior side slopes abruptly instead of gradually. 



In the next family are two genera, Sphaerium 

 and Pisidium. 



SphcBrium corneimi. — Very common. Shell some- 

 what globular, glossy, opaque, horn-coloured, 

 marked with lighter bands in the line of growth. 

 Usually on the bottom, but can suspend itself 

 by threads of mucus. 



SphcBvium rivicola. — Much larger. Also flatter 

 and more striated. Yellowish brown or greenish. 

 A whole series of young of different sizes will be 

 found in the animal. 



SphcBrium pallidum. — Local in canals and 



