This picture is of a goniatite on display at the Indiana State Museum. It is called Imitoceras rotatorium and labeled as being from the Rockford Limestone (Jackson County, Indiana). According to the Indiana Geological Survey website, this limestone layer was first referenced by Owen and Norwood in 1847 as "Goniatite limestone of Rockford". The website decribes the limestone as " typically gray, fine grained, argillaceous, ferruginous, and sparingly fossiliferous. It has a characteristic green mottling, weathers to a rusty brown, and is argillaceous or dolomitic in places, particularly in the upper part to the north. Echinodermal debris and other fossils, such as the cephalopods, are concentrated in places."
The Indiana Geological Survey also reports that the Rockford Limestone in different parts correlates to the Chouteau Limestone (Mississippi Valley), Meppen Formation (Illinois), Burlington Limestone and Fern Glen Formation. The Rockford Limestone is above the New Albany Shale but below the New Providence Shale.
A paper in the Journal of Paleontology from July 1942 entitled, A Goniatite from the Mississippian Fern Glen Formation of Illinois by A.K. Miller and Courtney Werner, says the genus in the past as been referred to as Aganides (by de Montfort in 1808), Brancoceras (by Hyatt in 1884), and finally Imitoceras (by Schindewolf in 1923).
This fossil is from the Mississippian Period.