This is the first time I have found this part of the trilobite Cheirurus niagarensis (Hall). It is the tail or pygidium section of the trilobite. It will be interesting to see how much of it is intact after it has been cleaned with sand abrasion. Fossil was found in the Waldron Shale of Clark County, Indiana, USA. It is dated to the Silurian Period.
Here is an illustration from the 1881 Indiana Department of Geology and Natural History Eleventh Annual Report by John Collett State Geologist Plate 34 Figure 16 Ceraurus (Chierurus) niagarensis trilobite pygidium.
Next picture is from a paper on Silurian trilobites.
Image from Figure 3 and 5 - photo by George Nelson
New and Old Silurian Trilobites from Southeastern Wisconsin, with
Notes on the Genera of the Illaenidae
by Percy E. Raymond
Bulletin of the Museum of Comparative Zoology at Harvard College