This is an exceptionally nice specimen of the Mississippian trilobite Ameropiltonia lauradanae. Any complete Mississippian trilobites are rare, and there is very limited access to the location where these are found, so there are very few of them out there on the market (this being the best I've seen).
It's .6" long and nicely centered on a 3.3x2.1" piece of limestone.
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.