When I saw this brachiopod I was amazed at the shape and proportions. It's a Stegerhynchus stricklandi from the Waldron Shale of Indiana. My first guess would have been a species of Unicinulus due to the overall shape of the shell and the exaggerated sulcus (the "U" shaped part on the anterior of the shell). I think the reasoning is that the brachial valve is so much deeper than the pedicle valve, whereas Unicinulus the valves are more equal in depth.
This specimen came from the Waldron shale which is Silurian in age (Wenlock epoch, Sheinwoodian to Homerian stage).