328 New Fresh-Water Shells. 
48° ; right side convex, flattened at the apex; left 
side very deeply concave ; suture deeply impressed ; 
aperture round-ovate, ludis with its upper: extending 
much beyond its lower margin. 
. Greatest breadth, .17 inch ; least breadth, .13 inch ; 
height, .06 inch. 
Cabinets of Bost. Soc. Nat. Hist.; of Middlebury 
College ; of S. S. Haldeman, of Marietta, Pa.; of J: 
G. Anthony, of Cincinnati ; and my own. 
Habitat. This species was discovered in the sum- 
mer of 1838, in a small spring in a rocky cavity, 
in South Boston. Nearly a hundred specimens were . 
obtained, and a much larger number were left. Vis- 
iting the same spot a few days since, (July, 1840,) I 
found. the spring filled up with stones to the top of 
the water, and not a shell to be seen. Last summer 
I obtained a specimen in Lake George, N. Y. Dr. 
Wm. Prescott has found the species in Lynn. 
Remarks. This species much resembles P. par- 
vus, Say, and for some time I doubted whether it 
were distinct. But the specimens uniformly differ 
from that shell in having the spire elevated above 
the plane of the last whorl, whereas in that species it 
is concave, and consequently this species is much 
more deeply umbilicated on the left side ; also, that 
species is distinctly carinate on the middle of the last 
whorl, but is reas tüdislinety carinate below the 
middle, if at all. 
f 
