This is a 1.65" long, stalk-eyed trilobite Asaphus kowalewskii. It is Middle Ordovician in age and was collected at the Vilpovitsy quarry in the St. Petersburg region of Russia. The stalk eyes likely allowed to survey it's surroundings while remaining buried in the mud.
Like nearly all of these asaphid trilobites from these quarries there is some shell restoration, as the shell tends to fracture, particularly on their tails when split.
Photo of the Vilpovitsy 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.