A partial skeleton of a mosasaur was collected from the upper Cretaceous Lewis Shale near Hesperus, Colorado in the northwestern San Juan basin. It is the third and most complete mosasaur specimen collected from the Lewis Shale. Several diagnostic skeletal elements were preserved including the right quadrate, part of the right maxilla and seven caudal vertebrae. The quadrate of the specimen has a suprastapedial process fused to the infrastapedial process. The teeth are massive, triangular in shape and circular in cross section. Three of the seven caudal vertebrae have fused haemal arches. Based upon the fused processes in the quadrate, the fused haemal arch of the caudal vertebrae, the shape and size of the teeth and the nature of the left mandible, the specimen has been identified here as a Prognathodon overtoni. The mosasaur collecting locality is estimated to be 21 m below the Huerfanito Bentonite Bed and leis within in the Baculites perplexus ammonite zone. Given the radiometric age of 75.76+/- 0.34 ma for the Hurfanito Bentonite (Fasset, et.al., 1997) and an average sedimentation rate of 11.25 cm/1000years) for this portion of the Lewis Shale, an age constraint of for this mosasaur was calculated to be 75.95 +/- 0.5 ma (middle to upper Campanian age.) The private landowner has donated the specimen to the New Mexico Museum of Natural History in Albuquerque, New Mexico.