28 PLANORBIS PARVUS. 



deep: aperture oval, oblique, longer than wide, 

 peritreme undeviating. 



COLOR of the shell, light wood-brown, some- 

 times with a tinge of greenish. 



GEOGRAPHICAL DISTRIBUTION. Very generally 

 diffused over the eastern, middle, and western 

 states; and found in the Northwest Territory. 



FOREIGN ANALOGUE. P. laevis. Gray's Tur- 

 ton, fig. 148. 



OBSERVATIONS. 



Except P. dilatatus, this is our smallest spe- 

 cies; individuals of a fourth of an inch in size 

 being of rare occurrence. Its smaller trans- 

 verse diameter, and the more open concavity 

 of the left side, distinguish it from small speci- 

 mens of P. deflectus, to which it is allied by a 

 tendency to deflect the last whirl towards the 

 left, and of which figure 20 presents an extreme 

 case. Individuals in which this character is 

 pretty well developed, constitute P. elevatus, 

 Adams. 



