This is a very large Sea Urchin Fossil mortality plate, Salenia schencki, from the Keasey Formation of Oregon. Around 11" long it features more specimens than you can count. Sea urchins are preserved intact in a beautiful orange color with white spines. 

The Keasey Formation preserves deep water fossils from around the end of the Eocene and the beginning of the Oligocene. These beautiful and rare fossils do not come on the market often, this specimen is from an old collection.