The somewhat water-worn specimen below is, I believe, Eospirifer radiatus. I found it among the cobbles and stones in a creek at Tunnel Mill near Vernon, IN. It's a little worse for wear but you can still see the fine radiating lines on the surface of the shell. That and the shell size are a hallmarks for the species.
The Tunnel Mill locality features the Waldron shale as a narrow band of fossiliferous rock that is frequently washed by the Muscatatuck River. By searching in the downstream gravels you can find fossils that have eroded out. The Waldron shale is Silurian in age (Sheinwoodian to Homerian).