4 PLANORBIS. 



whilst in P. armigerus exacutus and Segmentina 

 lineata, it appears only above, or on the right 

 side, as in Helix; but all these species have the 

 inclination of the aperture like other Planorbes, 

 or dextral Helices; and independently of any 

 inclination of the periphery to either side. 



As an additional argument in favor of the 

 aperture deciding the question, let us suppose 

 the apex and whirls of a very flat Helix, like 

 Polygyra septemvolva, to be pushed through; 

 and the heterostrophe coil will represent, not a 

 sinistral Helix, but a reversed Anostoma; which 

 genus resembles the typical Planorbes, in being 

 dextral, and having the aperture and apex upon 

 the same side. These being the facts of the 

 case, the apex must be left out of the question; 

 and taking the animal and aperture as our guide, 

 we must come to the conclusion that the shell 

 is dextral, and umbilicated above. 



The heart of Planorbis is placed upon the 

 right side, and the respiratory orifice upon the 

 left, contrary to what obtains in most pneumo- 

 branchiate mollusca; but M. Desmoulins ascer- 

 tained that the organs of digestion and genera- 

 tion are situated as in dextral shells; so that 

 the arguments drawn from the former fact, are 

 more than counterbalanced by the latter. 



