This Mississippian Period fossil appears to be a Dichocrinus striatus crinoid calyx plate. Fossil in image about 4 mm wide. It was found at Spergen Hill, Indiana at a famous Salem Limestone microfossil locality. The identification matches Figure 5b found on Plate XIII in the Indiana Department of Geology and Natural Resources Thirtieth Annual Report, W.S. Blatchely State Geologist, 1905.
Thanks to Kenny for sharing this fossil image.