Cope.) 512 [Jan, 18, 
From this table it may be seen that the passage from the small 0. 
gracilis to the large O. 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. 
eulbertsoni 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 O. afinis as large as that of the O. culbertsont 
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 coloradoénsis agrees in interorbital width with the 
small O. 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 not coincide 
exactly with the other measurements. The measurements of four skulls 
are as follows: O. gracilis No. 1, length of skull M. 114.5 ; No. 2, .180. 
0. coloradonésis No. 1, .129; No. 2, .185. 
Oreodon gracilis Leidy. 
Proceedings Academy Philada., 1851, 289; 1858, 392 ; 1854, 157 ; 1857, 
89; Owen’s Report Geolog. Survey, 1852, 550, Pl. XI, figs. 2-8 ; Pl. XIII, 
figs. 5-6. Ancient Fauna Nebraska 1853, p. 53, Pl. V, figs. 8-4; VI, figs. 
1-7. Extinct Fauna Dakota and Nebraska, 1869, 94, Pl. VI, figs. 2-8. 
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. .028..,.....4+ eueleve 0. g. gracilis 
Length of superior premolar series, M. .020........+. 0. g. coloradoénsis. 
Oreodon gracilis gracilis Leidy. 
Dakota, Nebraska and Colorado. 
Oreodon gracilis coloradoénsis Cope. 
Jolorado. 
Oreodon afffiimis Leidy. 
Extinct Mammalia Dakota and Nebraska, p. 105; Pl. TX, fig. 3. 
Probably from the White River beds of Nebraska. 
Oreodon culbertsoni Leidy. 
Owen’s Report Geological Survey, 1852, 548, Pl. X, figs. 4-6 ; XIII, 
figs. 8-4; Ancient Fauna Nebraska, Smithsonian Contrib. to Knowledge, 
1858, 45; Pl. Il, III, IV, figs. 1-5, V, figs. 1-2, VI, figs. 8-11; Proceeds. 
bi 
#) 
