This is a detailed, 2.1" long (if outstretched) specimen of the Corynexochida trilobite, Paralejurus. It still shows quite a bit of shell detail, and you can even see the hundreds of tiny eye facets under magnification. The limestone around it has been removed so that it looks like it is about to crawl off of the rock.
About 1% shell restoration along repaired cracks.
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.