250 0. a Marsh— Principal Characters of the Tillodontia. 



portion of the zygomatic arch. The lachrymal is of moderate 

 size, and is perforated by its foramen in front of the anterior 

 border of the orbit. The infra-orbital foramen is large. 



The palate is broad behind, narrow in front, and somewhat 

 excavated. The anterior palatine foramina are confluent, and 

 are enclosed between the premaxillaries and maxillaries. The 

 posterior palatine foramina are in the latter bones, near the first 

 premolars. The posterior nares are behind the last upper 

 molars. The occipital condyles are small, and sessile. The 



opening, partially occupied by the periotic. There was a dis- 

 tinct alisphenoid canal. 



The brain cavity in Tilhtheriiim is small, but proportionally 

 larger than in Dinoceras* The size of the brain compared 

 with the entire skull is shown in the accompanying cut, fig. 1. 



Figure 1. Outline of sku] 

 view. One-fourth natural size. 



As in most, if not all, Eocene mammals, the cerebral hemis- 

 spheres were small, and did not extend over the cerebellum or 

 olfactory lobes. The latter were large, and projected well for- 

 ward. The hemispheres were evidently more or less convo- 

 luted. There was no distinct tentorial ridge. The cerebellar 

 fossa is large, much expanded transversely, and elevated above 

 the cerebral cavity. There is a shallow pituitary fossa, and no 

 clinoid processes. The exit for the optic nerve is quite large. 



The adult dentition of Tilloiherium is represented by the fol- 

 lowing formula: 



Incisors _. canines — ; premolars _; molars—. 



