This is a section of polished one from the Pliosaur, Liopleurodon. One species of Pliosaur, has gained much notoriety and received the nickname "Predator X". The bone comes from the Middle Jurassic (160 million year old) Oxford Clay of England. Fragmentary bones are a fairly common occurrence in the clays of this formation and this one has been cut and polished on one side to reveal the beautifully preserved cell structure.
Liopleurodon is a genus of large, carnivorous marine reptile belonging to the Pliosauroidea, a clade of short-necked plesiosaurs. They were an apex predator during the Middle Jurassic period when seas covered Europe.
iopleurodon ferox, a pliosaur from the Late Jurassic of Europe, pencil drawing by Nobu Tamura (http://spinops.blogspot.com).