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.
Do you have Twitter or some other means that people can contact you? I live near multiple good cross-sections which include the Mahantango Fm. in West Virginia but didn't see a means to email you or anything.
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