PHYSA HETEROSTROPHA. 25 



to Saskatchewan." Mr. Nuttall brought speci- 

 mens from Lewis' river, Oregon; Mr. Couper 

 has found it on the Alatamaha, in southern 

 Georgia; and Dr. Mighels in Maine. 



OBSERVATIONS. 



This species may be distinguished from Physa 

 ancillaria by its greater length, darker color, 

 and in never having a very wide aperture. I 

 have examined hundreds from the Susquehanna, 

 and have never found one which could be con- 

 founded with the ancillaria, as it occurs in the 

 Delaware; although it is difficult to decide be- 

 tween those half-grown individuals in each, 

 which have a wide aperture in heterostropha, 

 and a narrow one in ancillaria; especially when 

 those specimens of the former are chosen which 

 have a rounded spire, as figure 8, plate 1. 



An extensive suite of purposely selected spe- 

 cimens might show the propriety of uniting the 

 two, but my present opinion is against such a 

 step. Some elongated individuals may be con- 

 founded with P. gyrina, but these must be con- 

 sidered an accidental variety, as I never saw 

 but two specimens, (figs. 6 and 7, plate 1,) 

 which are very long, and these I collected from 



