Platyceras is a genera of gastropods (contrary to how the name looks) that is very common throughout the lower Paleozoic. Most of the species have some kind of curvature to the shell that makes them look like snails like these that I detailed in a previous report on the Silica Shale.
Recently I found another species of this common genera in the fossil gravels I bought back from Arkona. It's called Platyceras conicum and you can see that it's most distinctive feature is the near absence of any shell curvature as one might expect.
You can see a little bit of curvature near the tip of the specimen but at first glance it looks totally straight. The specimen is a bit crushed but to me this looks like someone draped a piece of cloth over a tent peg. Were it not for my occasional perusals of the Friends of the UMMP Michigan Basin Image Archive I would not have come across this or this or this image that lead me to my ID.