Here is Discus catskillensis, the Angular Disc. I found this one at Ricketts Glen State Park. In most of my books it's listed as Discus rotundus catskillensis, but apparently it now enjoys full species status. It differs from D. rotundus by the angular periphery and lack of blotches.
As a bonus I got to actually witness this snail lay an egg through my microscope. It happened when I was cleaning out the little petri dish terrariums I've been keeping smaller species in. I looked at the snail and it looked odd, like it's neck was bulging. I placed it under my microscope and, sure enough, I was able to see it push out an egg. This was on Nov. 17 and they (there were 4 total) have not hatched yet.
|Range map from Land Snails of Limestone Communities and Update of Land Snail Distributions in Pennsylvania (Pearce.|