PLANORBIS. 239 



caused the keel to be considered as central, which it 

 is not. 



Dr. Turton's figure of Planorbis planatus certainly 

 represents this shell ; and probably he only described 

 a young specimen. 



Draparnaud and Michaud believe that the Helix 

 contortuplicata Gmelin, S. Nat. n. 144. (Planorbe en 

 vis, Geoff. 99. t. 3. f. 17, 18.) is only a monstrosity. 



Mr. Sheppard describes a monstrosity which had 

 the volutions nearly disjointed or pulled out. (Linn. 

 Trans, xvi. 157.) 



Mr. Alder observes : " I am not very sure, even 

 after the examination of Mr. Jeffreys' specimen, that 

 I perfectly understand the distinction between Pla- 

 norbis disciformis and P. carinatus. The degree of 

 carination is so very variable in different individuals 

 of the same species, that it is rather fallacious as a 

 distinguishing character." On reexamining the spe- 

 cimen here referred to, I could not find any character 

 of importance to distinguish them ; and Mr. Jeffreys 

 says, " they are often found living mixed with P. 

 carinatus." 



In the first edition of this work there was some 

 mistake about the reference to the figures, perhaps 

 occasioned by their being wrongly numbered by the 

 engraver. 



Planorbis marginatus should have been 87. and 

 not 88. 



Planorbis complanatus should have been 88. and 

 not 89. 



Planorbis carinatus should have been 89. and 

 not 87, 



