PLANORBIS TRIVOLVIS. 17 



scarcely touch the plane on which it lies; while, 

 in P. trivolvis, the shell would be lifted by the 

 lip; the aperture has not the sharp angle of the 

 left side, produced by the termination of the 

 carina, but in the young stages it is difficult to 

 distinguish the two." 



Professor Adams remarks that "P. lentus, 

 P. corpulentus, and P. trivolvis, of Say, are un- 

 doubtedly varieties of one species;"* but he 

 sent me large specimens of P. trivolvis, (pi. 2, 

 fig. 6,) as P. corpulentus; and believed the shell 

 now under consideration to belong to P. lentus. 

 I have figured it upon the same plate with the 

 latter, to afford a ready comparison between 

 them; and have thought best to describe it at 

 large, under a distinct heading. I have seen it 

 living in the vicinity of Boston, but have ex- 

 amined so small a number of individuals, that 

 I do not feel myself competent to make a final 

 decision between two authors whose location 

 gives them facilities which I do not enjoy. 



* Hist, of Vermont, Appendix. 



K 3 



