Paleozoic Cephalopod Fossil

This fossil is quite a rare find. A coiled cephalopod found it Clark County, Indiana which means it is probably either Silurian, Devonian or Mississippian in age. The label is not too specific. I would guess it is maybe 20 cm across in size.

It is on display at the Indiana University Southeast geology department as of August 2013. It was donated to them by John T. Shelton.