Clidastes

Clidastes is an extinct genus of mosasaur from marine environments of the late Cretaceous. 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.