This is a rare, 4.3" long Megistaspidella triangularis trilobite from the Middle Ordovician deposits near St. Petersburg, Russia. It's nicely prepared and it's caramel colored shell contrasts nicely with the light colored limestone. Both genal spines have been prepared free-standing.
Like nearly all of the trilobites from these quarries there is some restoration as the shell tends to flake when the rock is split.
Photo of the Putilovo quarry near St. Petersburg Russia where this trilobite was found.
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.