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Analysis of a stratigraphically-controlled sample of over 2,000 lagomorph specimens in the University of Nebraska State Museum, collected from sediments in the Brule Formation (Oligocene) of Sioux and Scotts Bluff Counties. Nebraska, indicates biostratigraphic and evolutionary changes in the lagomorph fauna throughout the Orellan of Nebraska.
A new species, Palaeolagus hemirhizis, described from the Orella A horizon, is intermediate between the typical Orellan species P. haydeni and the Chadronian species P. temnodon from Montana, Wyoming, and Saskatchewan. Palaeolagus haydeni and P. burkei show almost complete overlap in size range of cheek teeth. with the means of the measurements for P. burkei being slightly smaller than those of P. haydeni. Palaeolagus haydeni is known from Orella B through D, P. burkei from Orella C and D, and P. intermedius is known only from Orella D. Only Megalagus turgidus is known from all horizons of the Orella Member. Size change through the Orellan is demonstrated for P. burkei, P. haydeni and M. turgidus. Specimens of P. haydeni, P. burkei, P. intermedius, and M. turgidus are also known from the Lower Whitney Member (Whitneyan).