22 PLANORBIS EXACUTUS. 



Shell lenticular, fragile, translucent, and fine- 

 ly striate; slightly convex, but concave towards 

 the centre, and margined with a carina above: 

 convex, and widely umbilicated below: sutures 

 moderately deep: aperture very oblique, angu- 

 lated by the carina in front, and having the 

 middle of the right margin strongly arched. 



Color fight corneous. 



Geographical Distribution. Inhabits New 

 England, New York, and the western states. 



Foreign Analogue. P. nitidus, Gray's Tur- 

 ton, fig. 93. 



OBSERVATIONS. 



"It appears to be pretty closely allied to P. 

 nitidus of Europe, but it is larger, the umbilicus 

 much more dilated, and the aperture does not 

 embrace the penultimate whirl so profoundly." 

 — Say. Dr. Gould remarks that it diflTers from 

 the European analogue in having the aperture 

 "entirely below the sharp edge, instead of em- 

 bracing nearly an equal portion on each side, 

 as in that shell." Not having European speci- 

 mens, I have not been able to institute a com- 

 parison with ours. It is distinguished from P. 

 deflectus by the flatness of the whirls, and less 



