This is one of my favorite fossils in my small collection. It is a Dawsonoceras amycus? Silurian Period cephalopod shell fragment. It was found in the Waldron Shale of Clark County, Indiana. It has an added bonus with what appears to be a black crinoid holdfast attached to its shell. It might be Eucalyptocrinites crinoid.
The inside of the shell is now quartz. On the outside, sinusoidal lines still exist showing the growth lines of the creature. The shell is marked by distinct ridges (5 visible in this picture) that also set this specimen apart from other types of cephalopods I have found.
Using my Fossils of Ohio (Bulletin 70 Ohio Department of Natural Resources, Division of Geological Survey, Rodney M. Feldmann, Editor, 1996) reference book page 172 in chapter 14 entitled Phylum Mollusca, Class Cephalopoda, it says that cephalopod fossils have not been studied seriously in the United States in more than 50 years and only good identifications can be made on Middle Ordovician European fossils.
It goes on to describe the Dawsonoceras as "a longiconic nautiloid that is orthoconic or slightly curved. It is characterized by conspicuous transverse ridges called annulations."