This is a nice, enrolled Isotelus maximus from the famous trilobite beds at Mt. Orab Ohio. It it was outstretched it would be around 1.8" long. Both the genal spines are missing, but you rarely get enrolled Isotelus inflated like this, they are typically crushed.
Isotelus maximus is the the official state fossil of Ohio.
Trilobites were a very diverse group of extinct marine arthropods. They first appeared in the fossil record in the Early Cambrian (521 million years ago) and went extinct during the Permian mass extinction (250 million years ago). They were one of the most successful of the early animals on our planet with over 25k described species, filling nearly every evolutionary niche. Due in large part to a hard exoskeleton (shell), they left a excellent fossil record.