Cope. J b\2t [Jan. 18, 



From this table it may be seen that the passage from the small 0. 

 gracilis to the large 0. culbertsoni is accomplished by a series of inter- 

 mediate steps. That these extreme forms belong to one species cannot be 

 admitted without evidence of more complete transition than we yet 

 possess. As above remarked, groups of specimens represent each form 

 and adhere to the definitions given with considerable fidelity. The largest 

 of the specimens I refer to, the form 0. periculorum, however, reaches 

 .042 in the length of the true molar teeth, and the smallest of the 0. 

 culbertsoni measures .046. These I must consider as sub-species only. As 

 regards the three remaining forms the length of the true molar series 

 shows a- complete gradation. The size of the cranium, as indicated by the 

 interorbital width, is in the 0. affinis as large as that of the 0. culbertsoni 

 according to Leidy, and the combination of characters presented by this 

 form, would seem to entitle it to specific rank as suggested by Leidy. On 

 the other hand the form color adoensis agrees in interorbital width with the 

 small 0. gracilis, differing from it in the greater length of the muzzle and 

 of the cranium. But here, while the proportions of the premolar teeth 

 distinguish the forms well, the length of the brain-case does net coincide 

 exactly with the other measurements. The measurements of four skulls 

 are as follows : 0. gracilis No. 1, length of skull M. 114.5 ; No. 2, .130. 

 0. color adonesis No. 1, .129 ; No. 2, .135. 



Oreodon gracilis Leidy. 



Proceedings Academy Philada., 1851, 239 ; 1853, 392 ; 1854, 157 ; 1857, 

 89 ; Owen's Report Geolog. Survey, 1852, 550, PI. XI, figs. 2-3 ; PI. XIII, 

 figs. 5-6. Ancient Fauna Nebraska 1853, p. 53, PI. V, figs. 3-4 ; VI, figs. 

 1-7. Extinct Fauna Dakota and Nebraska, 1869, 94, PI. VI, figs. 2-3. 



Abundant in the White River beds of Dakota, Nebraska,, Colorado and 

 "Wyoming. 



The two sub-species are distinguished as follows : 



Length of superior premolar series, M. .023 O. g. gracilis 



Length of superior premolar series, M. .029 0. g. color adoensis. 



Oreodon gracilis gracilis Leidy. 

 Dakota, Nebraska and Colorado. 



Oreodon gracilis coloradoensis Cope. 

 Colorado. 



Oreodon affinis Leidy. 

 Extinct Mammalia Dakota and Nebraska, p. 105 ; PI. IX, fig. 3. 

 Probably from the White River beds of Nebraska. 



Oreodon culbertsoni Leidy. 



Owen's Report, Geological Survey. 1852, 548, PI. X, figs. 4-6 ; XIII, 

 figs. 3-4 ; Ancient Fauna Nebraska, Smithsonian Contrib. to Knowledge, 

 1853, 45 ; PI. II, III, IV, figs. 1-5, V, figs. 1-2, VI, figs. 8-11 ; Proceeds. 



