16 PLANORBIS TRIVOLVIS. 



appears, when the inner portion of the last 

 whirl revolves to the right of the carina, as in 

 figure 3; in this case, the right margin of the 

 aperture is nearly level with this side of the 

 shell, but it is frequently thrown below, or to 

 the left of it, when it bears some resemblance 

 to figure 5: aperture slightly compressed ante- 

 riorly, the left margin extending beyond the 

 plane of the shell. 



Color light brown, sometimes greenish. 



Monstrosity. Posterior extremity of the 

 foot divided. 



Geographical Distribution. Massachusetts, 

 Lake Erie, Indiana. ? 



OBSERVATIONS. 



In color and consistency, the ova resemble 

 those of P. bicarinatus. Dr. Gould has ex- 

 pressed an opinion, that if this be not P. lentus, 

 it must be an uncharacterized species. He re- 

 marks that it is "a darker shell than P. trivolvis, 

 and is distinguished from it by its left side and 

 its aperture. The cup of the left side is less 

 smooth and regular, and is not bounded by the 

 sharp, elevated line; when this shell is laid upon 

 its right or upper side, the lip of that side will 



