236 



Shell Life 



species are 

 been kind 



Common River Snail 

 (one-half nat. size) 



herbivorous mollusks. The males are smaller than 

 the females, because the latter retain their eggs until 

 they are hatched, and, therefore, require more roomy 

 whorls for their accommodation. The two native 

 much alike superficially, but nature has 

 to the tyro in giving them marks that 

 serve to distinguish them readily. 

 The Common River Snail (F. vivi- 

 para) has a thicker and longer 

 shell, a blunter apex, less swollen 

 whorls, a less circular mouth, and 

 the umbilicus is reduced to a small 

 narrow chink. Lister's River 

 Snail ( 1 ^. contecta) is a little larger 

 than the other, has a more circular 

 mouth, and a distinct and deep though small um- 

 bilicus. It is a much more local species than the 

 other. They are both fairly active 

 animals, and tolerably hardy, for 

 V. contecta ranges from Finland to 

 the Pyrenees, and F vivi}XLra has 

 been known to produce a batch of 

 young shortly after having been 

 thawed out from a temperature of 

 23° F. 



The last of these operculate fresh- 

 water species we have to glance at 

 are the so-called Valve-shells (Valvata), of which 

 we have two — so distinct in form that there is no 

 difficulty in identifying them. The animals are 

 built much on the same plan as the foregoing — the 

 head drawn out into a long snout, and the eyes at 

 the base of the tentacles, but in this case on the 



Lister's River Sric 

 (one-half nat. size) 



