UNGULATA. 



297 



young individuals, but most of them are closed in the adult. It is clear 

 that the parietals are relatively large, while the frontals are short. The 

 nasals are excessively elongated, being about half as long as the entire 

 skull : and at their anterior end there is a pair of minute prenasal ossifica- 

 tions, apparently homologous with the ossicle sometimes found at the 

 extremity of the snout in the existing pigs. The jugal bone of the 

 zygomatic arch extends well forwards to the front of the orbit, where it 

 meets an extensive lachrymal element. The maxilla forms a large portion 

 of the lateral surface of the skull, and bears all the upper teeth; it is 

 perforated by a large antorbital foramen, which is obscured in direct 



FIG. 170. 



Tinoceras ingens ; restoration of skeleton by 0. C. Marsh, one-thirtieth nat. 

 size. M. Eocene (Bridger Formation) ; Wyoming. 



side view (fig. 170) by a ridge extending upwards in front of the orbit. 

 The premaxillae are slender and toothless, but do not extend further 

 forwards than the overhanging nasals. The mandible is comparatively 

 slender, except at the symphysial end, where the two mini are fused in a 

 massive symphysis, deeply excavated above, and where a pair of decurved 

 processes protect the upper canine tusks. The condyles for articulation 

 with the upper jaw are remarkable (unique among Ungulata) in being 

 directed backwards, not upwards, and in being raised very little above the 

 level of the grinding surface of the lower molars. The coronoid region is 

 much elevated, and the angle of the mandible is sometimes distinctly 



