12 PLANORBIS GLABRATUS. 



from Mexican ? shells, and fig. 2 from a New 

 Orleans specimen. Mr. Nuttall has placed an 

 individual in the Academy's collection, which 

 he found in Oregon. 



OBSERVATIONS. 



My shells do not agree perfectly with Say's 

 description, copied above; yet they most proba- 

 bly constitute the species he had in view. They 

 differ from P. trivolvis by having a much more 

 oblique peritreme, the whirls more nearly cylin- 

 drical, the diameter increasing less rapidly, and 

 without any tendency to carination upon the 

 left side. 



