PLANORBIS. 31 



7. P. VORTEX (whirlpool). 



Very flat, glossy, yellowish horn-colour ; whorls six to 

 eight ; sharply keeled. 



Found in similar habitats to the last species, but is 

 less common. I have noticed that it is much more 

 often found with a perfect epidermis than P. spirorbis, 

 even in the same pond as that species. 



It may be distinguished from P. spirorbis by its 

 prominent keel ; it is also flatter and thinner. 



Var. compressor. Thinner and much flatter, and 

 more sharply keeled. 



8. P. CAKJNATUS (keeled). 



Compressed, spire sunk, only slightly convex beneath, 

 slightly glossy, semi-transparent, pale brown ; whorls five 

 to six ; suture deep ; mouth angulated above and below ; 

 sharply keeled in the centre of the outer margin. 



Found in the south and east of England, in Lan- 

 cashire, south of Scotland, in Wales, and Ireland ; 

 but it is local, and not plentiful. 



Var. disciformis. Flatter and thinner. 



9. P. COMPLANATUS (flattened). 



Concave above, flat beneath, nearly opaque, strongly 

 keeled below ; whorls five to six ; mouth rhomboidal. 



This species is often so like the last that a very 



