Few snails are as well known in the Devonian as Platyceras is. The name sounds similar to that of an Ammonite since they share a similarly pronounced last part of their names. While they are both Mollusks, Platyceras is a Gastropod (or stomach foot) wile Ammonites are Cephalopods (head foot).
Platyceras is a very prolific genera within the Silurian to Devonian and is known to exist from the Late Ordovician (Sandbian - 460 mya) through the Permian (Capitanian - 260 mya). Within the Silica Shale they can grow to be quite large and I've found a couple of species:
Platyceras bucculentum (Hall 1862) - Note the undulating, wide, bell like margin. This species often has a well defined crest but this specimen is lacking that.
Another Platyceras bucculentum
This specimen appears to be a Platyceras rarispinum based on the spine bases present on the shell.
All ID's are based on the 1975 book by Robert V. Kesling & Ruth B. Chilman: Strata and Megafossils of the Middle Devonian Silica Formation: Museum of Paleontology, Papers on Paleontology, No. 8, pg. 117, pl. 19,20.