192 PLANORBIS. 
4, PLANORBIS GRACILENTUS, Gould. 
Plate 5, fig. 7, 8, 9. 
Shell discoidal, 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. Liebmanni, of Dunker, which is 
very different and has not been found west of 
the dividing ridge of mountains which so 
clearly separate our mollusca into two faunas. 
(39) 
