This fossil appears to be a Dawsonoceras americanum cephalopod. It was found the Osgood Formation at Napoleon Quarry in Ripley County Indiana USA. It dates to the Silurian Period.
Used The Silurian Fauna of Kentucky by August F. Foerste for a list of fossils found in the Osgood Formation to narrow down the species. It can be found at http://www.uky.edu/OtherOrgs/KPS/poky/files/pokych03-01-46.pdf
