OP THE UNITED STATES. 16d 



equidistant, parallel, slightly elevated lines ; spire 

 a little convex; volutions four: aperture longer than 

 wide; umbilicus exhibiting all the volutions. 



Breadth, less than 3-20 of an inch. 



Inhabits Upper Missouri. 



This shell has evidently the habit of a Helix, and 

 may probably belong more properly to that genus. 

 but having found it only in a dried up pond, in com- 

 pany with a vast number of aquatic shells, I refer 

 it for the present to this genus. 



8. P. *exacuous. Dextral, depressed, with an 

 acute edge. 



Inhabits Lake Champlain. 



Cabinet of the Academy. 



Shell depressed ; ichorls four, striated across, 

 wider than long, not elevated above the suture, but a 

 little flattened, sides obliquely descending to an acute 

 lateral ed^e, below the middle ; spire not impressed; 

 suture not profoundly indented; beneath, body whirl 

 flattened, on the inner edge rounded ; umbilicus re- 

 gular, exhibiting all the volutions to the apex ; aper- 

 ture transversely sub-triangular ; labrum angulated 

 in the middle, arquated near its inferior tip, the su- 

 perior termination just including the acute edge of 

 the penultimate whorl. 



Greatest Breadth, rather less than 1-4 of an inch, 



This species was found in Lake Champlain by 

 Mr. Augustus Jessup, who deposited it in the col- 

 lection of the Academy. Only two specimens oc- 



