This is a very large example of the stalked eyed trilobite Asaphus punctatus. At 3.8" long it quite large for the species, is laid out nice and prone and is beautifully preserved. The preparation work to remove the limestone surrounding it was very well done. It comes from the Gostlitsy Quarry in the St. Petersburg region of Russia.
The shell had fractured pretty badly during collection which is common with Asaphids, so there is a lot of repair to piece the shell back together but the actually amount of restored shell looks pretty minimal.
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.