"Pond-snails 325 



regarded as two species by tlie separation of some 

 well-marked individuals as P. lineatnfi. Tlie sliell is 

 (|noit-8haped, convex above, almost flat beneath, with 

 a central depression on both sides ; semi-transparent, 

 highly polished, of a reddish or yellowish 

 liorn colour, and bluntly keeled. The body- 

 Avhorl clasps the preceding one so exten- shinmg 

 sively that only about one-third is visible. ^""^p^^ 

 Mouth heart-shaped, umbilicus small. In the form 

 known as linfatu>^ the body - whorl is partially 

 divided inside by four or live transverse plates, 

 which show throuo-h the shell as white lines. Some 

 authors separate this species from Planovhis under 

 the name of Segnientina idtiiht. It inhabits small 

 pools of stagnant water, where it feeds upon decaying 

 vegetation ; the seo-mented form beino; more local 

 than the other. 



The fresh-water snails are brought 

 to a close with the Bladder-snails 

 (PJtysa), of which Ave have two species. 

 They have very thin and highly 

 polished shells, with spires turnin< 

 from right to left (siniMral). The 

 animal has two long slender tentacles, with 

 eyes at their base. The Fountain Bladder- 

 snail (P. fontinalis) has a shell nearly half 

 an inch long, much like that of a short- gheii of 

 spired Limvcecc except that there is no epi- Fountain 

 dermis, and the mouth is to its right instead 

 of the left. But the characteristic feature of this 

 species is its mantle, whose sides are expanded so 

 that they turn up and wrap the shell. The borders 

 of these mantle-expansions run into finger-like lobes 



Fountain Bladdf "*'" "" '" ' •■---- ,^.^^^^., ^ ^^ 



