I am going to preface this with a definitive statement. I know nothing about non-Devonian gastropods.
I obtained a rather nice shell collection recently with hundreds of really quality shells. As I was sorting through them, I came across this one which would appear to be fossilized. It has a very matte, chalky texture, and shows signs of having spent some degree of time not alive. That being said, I will restate my previous assertion that I know nothing about non-Devonian gastropods. From some cursory research via image search, it would seem to be a Calotrophon.
If anyone has any thoughts, that would be super! Thanks in advance!
P.S. I apologize for the photo quality. My phone has pretty pathetic depth-of-field at close range.
-J