Clidastes

Clidastes is an extinct genus of mosasaur from marine environments of the late Cretaceous.Cope E.D. 1869. On the reptilian orders, Pythonomorpha and Streptosauria. Proceedings of the Boston Society of Natural History 12: 250–266. It was an agile and fast swimmer which cruised the surface or shallow waters hunting for prey.

Clidastes was one of the smallest of the mosasaurs, averaging  in length, with the largest specimens reaching  long. 
It had a delicate, slim form with an expansion near the tip of the tail. This enabled it to chase down the fastest of prey.

 References 

Category:Mosasaurs