This is a Fossil of a Polished Mosasaur Vertebra. This specimen was found in North Sulphur River, Texas.  This piece measures approx. 46 x 36 mm: thickness is 24 mm: weight is 67 grams. 
 
Mosasaurs are a family of enormous marine reptiles that are extinct and ruled the seas for 25 million years during the Cretaceous period. Some species including the Mosasaurus could reach lengths of 50 feet or more.