Bargain, 3.1" Dicranurus Trilobite - Atchana, Morocco
This is bizarre trilobite looks like something you might see in one of the Alien movies. It's fittingly name Dicranurus monstrosus. While fakes of this trilobite abound, real examples like this one are fairly rare.
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 currently described species, filling nearly every evolutionary niche. Due in large part to a hard exoskeleton (shell), they left an excellent fossil record.