376 



MAMMALIA. 



Tillotherium (fig. 213). The cranial region of the skull is elongated, 

 with a sagittal crest, and the brain-cavity is relatively small ; the facial 

 region is short and slender. The zygomatic arch curves considerably 

 outwards, and the hinder limits of the orbit are not marked by any 

 postorbital process either from this arch or from the frontal. The auditory 

 bullse are relatively small. The powerful premaxillee extend backwards 

 upon the superior surface of the skull, between the maxillse and slender 

 nasals, almost as far as the frontals. The rnandibular rami are fused at 

 the symphysis and the convex articular facette is raised high above the level 

 of the teeth, not quite so much elevated as the coronoid process. The 

 jaw cannot have been capable of the antero-posterior movement character- 



.,.,, m , , , , ,, , . i. 2, c. 1, pm. 3, m. 3 _,, 



istic of the rodents. The dental formula is : -^-- - . The 



i. 2, c. 1, pm. 2, m. 3 



FIG. 213. 



Tillotherium fodiens ; skull and mandible, left lateral aspect, one-quarter nat. size. 

 M. Eocene (Bridger Formation) ; Wyoming. (After Marsh.) 



molars and premolars are brachyodont, and well covered with enamel ; 

 those of the upper jaw are distinctly tritubercular in pattern, while those 

 of the lower jaw approach the same condition, but are rather of a cutting 

 type (tuberculo-sectorial). The canine in both jaws is insignificant, 

 interrupting the short diastema between the premolars and the incisors. 

 One pair of incisors, evidently no. 2 of the typical mammalian series, is 

 much enlarged in each jaw, growing from persistent pulps and enamelled 

 only on the anterior face. The second pair of upper incisors (no. 3) is 

 quite rudimentary, and flanks the enlarged pair, is not placed internal to 

 them. The distal end of the humerus is expanded, with an entepicondylar 



