508 University of California Publications in Geology [Vol. 11 



Only small portions of the skull are present in the type specimen, 

 no. 21350 (fig. 84). The anterior portion of the base of the zygo- 

 matic arch arises above M 1 . Anterior to the root of the zygomatic 

 arch the maxillary appears to be somewhat flattened superiorly in a 

 plane nearly parallel with the palate. The infraorbital foramen is 

 situated at the posterior end of this flattened area, and above the 

 anterior half of M 2 . In specimen 21485 (figs. 85a and 85fe), the base 

 of the zygomatic arch is shown more satisfactorily. The lower jaw 

 as seen in several quite fragmentary specimens has approximately 

 the form shown in Merycochocrus. It is high anteriorly and on one 

 specimen seems to increase in height rapidly below the posterior 

 molars. It does not seem to show the unusual elevation of the 

 posterior region of the mandible exhibited in Pronomotherium. 



The dentition is characterized by height of the crowns of the 

 cheek-teeth, relatively great length of the molar series compared 

 with the premolars, advanced specialization of the premolars, and a 

 tendency to crowding of the premolars anteriorly. 



The stage of complication of the premolars is near that of Mery- 

 cochoerus proprius, and not more advanced than in that species. In 

 the Barstow species the elongation of the cheek-tooth crowns seems to 

 have advanced farther and the external anterior and posterior styles 

 in P 3 and P* seem better developed than in M. proprius. In the 

 Barstow form the outer faces of P 3 and P 4 are nearly flat with a faint 

 longitudinal rib, which the writer has not seen on M. proprius. 



There are suggestions of external paracone and metacone ribs on 

 the upper molars of the type specimen of M. buivaldi. These ribs are 

 not clear on no. 21485. 



On P 2 of M. proprius there is a small anterior pocket in the enamel 

 which does not appear on the only Barstow specimen showing this 

 tooth. An internal basal cingulum is clearly shown on the upper 

 premolars of the Barstow form. It is faint or absent on the upper 

 molars. 



The upper canine is triangular in outline. The upper incisors are 

 very small. 



In the lower dentition the degree of complication is approximately 

 as in M. proprius, and not more advanced. P x is relatively smaller, 

 or the other premolars relatively larger than in M. proprius. 



P 3 and P 4 of M. buwaldi are both less complicated than in M. 

 rusticus of the Pawnee Creek beds of Colorado (Am. Mus. Nat. Hist., 

 no. 9115, now in collections of University of California). The antero- 



