This piece was found in southeastern Maine. The last thing on my mind at the time was any sort of fossil. The area is granite ledges split by basalt dykes interspersed by cobble beaches of similar rock type. My turning point on the trip was the end of a cove that is pounded especially hard by the winters storms. The cobble is piled 20 feet high in a wall 60 feet wide at the base across the cove. As I got back slightly in the lee of some boulders I noticed the snail in what I recognized as the Pleistocene clay of the Presumpscot formation. I only noticed the other side after I got it home.
It's not mineral. Fails a tap test miserably.
The snail is on the under side in the other photos.
What the heck is this thing ?