Clidastes

Clidastes is an extinct genus of mosasaur lizard from marine environments of the Late Cretaceous. Clidastes was the smallest of the mosasaurs, averaging 2-4 meters (7-12 feet) in length, with the largest specimens reaching 6.2 meters (20 feet) long.

Description
Clidastes was an agile and fast swimmer that cruised the surface or shallow waters hunting for fish, flying reptiles and anything that got too close. It possessed a delicate and slim form with an expansion of the neural spines and chevrons near the tip of the tail and this enabled it to chase down the fastest of prey.