The earliest known occurrences of the species are middle Santonian in age and from the Niobrara Formation of Kansas, whilst the latest are Middle to Late Campanian in age, coinciding with a poorly understood middle Campanian intercontinental mosasaur extinction event, which seems to have heavily affected genera such as Clidastes. Russell (1967) listed the following unambiguous character states for the species: "Premaxilla "V"-shaped in horizontal cross-section, small. rostrum present anterior to premaxillary teeth. Posteroventral portion of root of second premadlary tooth exposed on sutural surface with maxilla. Premaxillo-maxillary suture rises posteriorly in gentle curve to terminate at point above seventh maxillary tooth. Premaxillary suture of maxilla smoothly keeled and paraIIels longitudinal axis of maxilla. Sixteen-18 teeth in maxilla. Median dorsal surface of parietal moderately broad. Parietal foramen smalI, lies close to suture with frontal and opens ventrally into elliptical excavation in parietal, length of which exceeds that of dorsal opening by about five times. Anterior border of prootic forms shelf beneath prootic incisure, then descends abruptly to basisphenoid. Foramen for cranial nerve VII leaves brain cavity through medial wall of prootic. Infrastapedial process present on quadrate. Seventeen to eighteen teeth in dentary.". Russell (1967) also referred a large number of fragmentary species of Clidastes to C. propython on the basis of that those with good cranial material were morphologically indistinguishable from the type specimen of C. propython. Among these former species now seen as synonyms of C. propython are C. "cineriarum", C. "dispar", C. "velox", C. "wymani", C. "pumilus", C. "tortor", C. "vymanii, C. "stenops", C. "rex", C. "medius" and C. "westi". Clidastes iguanavus The Campanian C. iguanavus is the original type species of Clidastes and poorly known in comparison to C. propython and C. liodontus. The type specimen consists of a single vertebra from the anterior thoracic region, YPM 1601, collected in a marl pit near Swedesboro, New Jersey. The vertebra is similar to that of the other species in its general proportions and the strong zygosphene-zygantrum articulation. C. iguanavus can be differentiated in its central articulations, which are kidney-shaped in outline, with a stronger emargination dorsally for the spinal cord, and in the relatively stout proportions of the centrum. References Callison, G. (1967). Intracranial mobility in Kansas mosasaurs. Lawrence Charles H. Sternberg Transactions of the Kansas Academy of Science Vol. 30 (Apr. 18, 1919 - Feb. 19, 1921), pp. 119–120 Cope, E.D. 1868. On new species of extinct