This is a mosasaur tooth in a display box and accompanying information label. While it does have some wear, as pictured, this does not detract from this great example.
 
Despite what has been portrayed in popular movies, Mosasaurs were not dinosaurs, but rather aquatic reptiles  more  closely related to monitor lizards . The largest is estimated to reach nearly 50 feet, while others were far more modest in size, measuring between 3- 13 feet. 
 
3.5cm x 2.1cm