Mesolimulus sp.
This is a fossil of the horseshoe crab Mesolimulus from the Solnhofen limestone in Germany. It lived during the Late Jurassic period. Horseshoe crabs are not related to crustaceans (crabs), but rather to spiders and scorpions.
Excavation site
I bought this at a fossil shop when I visited Solnhofen, Germany, right at the excavation site. They said it took 20 hours to prepare (clean) the fossil.