EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



381 



brachycepJialus (Am. Mus. 1495). The general contour 

 of the jaw in these two species, however, is somewhat 

 similar; there is the same long, slightly convex chin; 

 the lower border in profile is convex below the grinders, 

 slightly concave below the coronoid, and is produced 

 downward and backward into the angle, a concave 

 border rising from the angle to the condyle; and the 

 stout, somewhat recurved coronoid processes have a 

 heavy anterior and gently convex anterior border. 



This jaw, as compared with earlier forms, certainly 

 resembles in its main features and proportions those 



vertically oval extension for articulation with the 

 postglenoid process. The broadening and downward 

 extension of this postglenoid facet on the inner side 

 of the condyle is a striking progressive feature, which 

 was probably acquired by all late Eocene and lower 

 Oligocene titanotheres. The anterior border of the 

 rotula extends nearly straight across, as in Manteo- 

 ceras and probably also as in other phyla. The 

 posterior border of the jaw arches gently forward below 

 the condyle and then suddenly expands backward into 

 the downward and backward produced angle. 



Figure 321. — Lower jaw of ProUlanotherium superbum 



One-fourth natural size. Am. Mus. 2501 (type), reversed; White River, Uinta Basin, Utah; Uinta C. The coronoid is somewhat altered by 



crushing. 



of P. emarginatum, Manteoceras manteoceras, and Mesa- 

 tirMnus megarhinus. 



The symphysis is very massive, extending 158 

 millimeters anteroposteriorly, with 100 millimeters as 

 the least transverse measurement of the chin; it is 

 gently convex anteroposteriorly and somewhat more 

 decidedly convex transversely; the postcanine con- 

 striction is relatively less decided than in M. manteo- 

 ceras; the jaw increases rapidly in depth from 109 

 millimeters behind ps to 124 behind m2 and 163 

 behind m^. In view of the relatively short diastema 

 behind the canine and the rather rapid rise of the 

 coronoid border behind ms, it appears that this jaw 

 is progressively shortening and deepening. The 

 coronoid is stout, gently recurved, and relatively less 

 elevated above the postcoronoid border than in 

 M. manteoceras or Brontops hracJiycephalus. 



The condyle is greatly extended transversely 

 (106 mm.) and narrow anteroposteriorly (24 mm.), 

 therefore exhibiting a rather sharply convex rotula, 

 except on the inner side, where it exhibits a deep, 



Measurements of type loioer jaws of ProUlanotherium and 

 Brachydiastematheriiim, in millimeters 



Is, anteroposterior 



I3, transverse 



C, maximum anteroposterior 



diameter 



C, maximum transverse diameter. 



C, lieight of crown 



Postcanine diastema, maximum. _ 

 Postcanine diastema, minimum. _ 



Pi-p4, anteroposterior 



Pi, anteroposterior 



P2, anteroposterior 



P2, transverse * 



Pj, anteroposterior 



P3, transverse ' 



Tt, anteroposterior 



P), transverse ' 



Ml, anteroposterior 



Ml, transverse ^ 



105 



« Estimated. ^ Transverse measurements are made through anterior lobe. 



