192 PLANORBIS. 



4. PLANORBIS GRACILENTUS, GouJd. 



Plate 5. fig. 7, 8, 9. 



Shell discoid al, compressed, finely striated ; 

 right side flattened ; left side moderately con- 

 cave ', on each side four rounded whorls are 

 visible ; the last obtusely angular at the peri- 

 phery; aperture quite oblique, wide, round- 

 edly oval. 



Diam. 12-5, alt. 3 mill. 



No North American species of equal size 

 can be compared with this well-marked, wheel- 

 shaped species. Mr. Binney has confounded 

 with it PL Liehmanni, of Dunker, which is 

 very different and has not been found west of 

 the dividing ridge of mountains which so 

 clearly se23arate our mollusca into two faunas. 



(39) 



