Holotype: UPI R 163, a nearly complete skeleton described by Wiman (1920).
Type locality and horizon: Smoky Hill Member, Niobrara Chalk, Santonian (Late Cretaceous), Kansas (USA).
Referred specimen: USNM 3777, incomplete skeleton, Smoky Hill Member, Niobrara Chalk, Santonian (Late Cretaceous), Beaver Creek, Logan County, Kansas (USA) (Russell, 1967: 126).
Emended diagnosis: Ambiguous characters: premaxilla–maxilla lateral suture ending posterior to the 9th maxillary teeth; tail about 40% of the head and trunk length (convergent in mosasaurines); caudal vertebra length greater than width; fewer than four pygal vertebrae; femur length about twice distal width (convergent in Clidastes). Autapomorphies: parietal with smooth triangular table extending very far posteriorly, bearing medium-sized circular foramen, located at distance twice its diameter from the frontal–parietal suture, and surrounded anteriorly and posteriorly by two parallel ridges; rounded quadrate with regularly convex tympanic ala; vertebral formula: seven cervicals, 24 dorsals, four pygals, 28 median caudals and at least 41 terminal caudals; humerus length approximately 2.5¥ distal width.