DAKOTA AND NEBRASKA. 105 



the tertiary dej^osit of the Mauvaises Terres of White River, or bed A of his table, as 

 indicated on page 20. Their associates are the remains of Titanotlierium Prouti, 

 Hyopotamihs americaims, Lopliiodon occidenlaUs and Rhinoceros occidental is. No 

 remains of Oreodon Calhertsoni, 0. gracilis and 0. major were found with them. 

 Rhinoceros occidentalis is common to the beds containing the latter and the former, 

 Hijopotamus and Lophiodon are found in the eocene formations of Europe. These 

 facts indicate Oreodon affinis, 0. hyhridus and 0. bullatus to be species which pre- 

 ceded the others in time, and were perhaps their ancestors, from one or another of 

 which they may have been derived, according to the doctrine of natural selection, 



Oreodon affinis. 



The specimen now referred to a species with the above name, was previously 

 noticed under No. 10, page 96, as a variety belonging to Oreodon gracilis. It is 

 represented in figure 3, plate IX, and consists of the facial portion of a skull, which 

 in size is as lai-ge as the corresponding portion of some skulls of Oreodon Culbertsoni, 

 but which exhibits more of the peculiarities of Oreodon gracilis. As in this and diifer- 

 ing from the former, the orbits are proportionately small and the lachrymal fossae 

 shallow. Likewise, the posterior ends of the nasals together form a half-ellipse, as in 

 0. gracilis, but even more blunt than usual in this species. The teeth, though larger 

 than is commonly the case in the latter, are yet smaller than in some specimens, 



Oeeodon hybridus. 



Figure 4, plate IX, represents a specimen nearly corresponding with the former 

 one, but which in its proportions approaches Oreodon major, while the teeth are no 

 larger than usual in Oreodon Gulherisoni. The face above is moi'e convex trans- 

 versely than in the adult skull of 0. major, or is less abruptly vertical at the sides. 

 The orbits have the same proportions as in the latter, but the lachrymal fossas are 

 less deep and resemble more those of 0. gracilis. The nasal bones are narrower than 

 in the specimens referred to 0. major, and terminate posteriorly as in a young skull 

 of the latter and nearly as is usual in 0. gracilis, but their lateral borders, between 

 the very blunt angular processes of the frontals, are sigmoid. The molar teeth indicate 

 an old animal, as their crowns are much worn. Their size, as before mentioned, is 

 not greater than is usual in 0. Gidbertsoni. 



Measurements of the two preceding specimens, in comparison with corresponding 

 parts of 0. gracilis, 0. Gidbertsoni and 0. major, are as follows ; 



X4 



