336 0. a Marsh— Principal Cho 



; of the 



like the middle pair ia Dinoceras, the nasals forming only the 

 inner margin of the base. These protuberances are placed 

 transversely, as in modern Artiodactyles, and extend upward 

 and outward. They vary much with age, and probably dif- 

 fered with the sex. There are large air cavities in the base of 

 these horn- cores. The nasal bones are greatly developed, and 

 firmly co-ossified. Their anterior extremities are produced, and 

 overhang the large narial orifice. The premaxillaries are dim- 

 inutive, and do not usually extend forward so far as the end of 

 the nasals. The infra-orbital foramen is very large. The lachry- 

 mal forms the anterior border of the orbit. The latter is small, 

 and continuous with the elongated temporal fossa. There is no 

 postorbital process on the frontal. The zygomatic arches are 

 massive, and much expanded. The malar extends forward be- 

 yond the lower margin of the orbit. The zygomatic process of 

 the squamosal is elevated, and more or less incurved above. 

 There is a large postglenoid process, which forms the anterior 

 border of the external auditory meatus. The latter is bounded 

 behind and below by the post-tympanic process of the squamosal. 

 There is a large par-occipital process. The occipital condyles 

 are large, and well separated. Their position indicates that the 

 head was declined when in its natural position. There is a 

 large condylar foramen, and a distinct alisphenoid canal. The 

 palate is deeply excavated, especially in front. The posterior 

 nares extend forward between the last upper molars. 



The brain cavity in Brontotherium is small, and its form is 

 shown in Plate XI, the figures of which are drawn from a nat- 



)-tenth natural size, 

 urai cast of the brain-case of B. ingens Marsh. The size of the 

 entire brain compared with that of the cranium is shown m 

 the accompanying cut, figure 2. 



