This is a large, fantastically-prepared Comura bultyncki which would be around 3" long if it was outstretched. It is quite large for the species and is a top quality specimen. Just check out the shell detail including eye facets and dozens of free-standing spines. Probably 60-80 hours of prep work went into this marvelous specimen. There is a natural calcite seem running through middle the specimen which adds to it's character.
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.