PALUDINAS. 



101 



hundred eggs, in numerous clusters. It is further 

 distinguished from its specific brethren in not being 

 confined to a purely vegetable diet, but occasionally, 

 sometimes frequently, indulging in a meal off a dead 

 fish, or even brother or sister. 



The genus Planorbis is not geologically older than 

 Limnea> both having an antiquity which dates back 

 from the early tertiary period. We have eleven 



Fig. 49. 



Planorbis corneus. 



British species, most of which are to be met with in 

 every river, canal, pond, or tarn. They vary consider- 

 ably in size, some being scarcely larger than a pin's 

 head ; whilst Planorbis corneus often measures three- 

 quarters of an inch across. All of them are herbivor- 

 ous, and have equally the same habit, when crawling, 

 of lugging their shells behind their half-extended 



