idiophyllum

from The Century Dictionary.

• noun A genus of fossil ferns established by Lesquereux, based on a small round or broadly obovate leaf found at Mazon Creek, Illinois, in the lowest strata of the middle coal-measures. This leaf by its peculiar areolation is closely related to Dictyophyllum, but differs from it in not having the pinnate character which the leaves of all the species referred to that genus have.