EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



539 



3. The zygomatic arches spread greatly in males, 

 less in females; the zygomatic index becomes very 

 high in extreme forms; the postglenoid unites with 

 the post-tympanic process to inclose the external audi- 

 tory meatus; the transverse measurements of the su- 

 perior grinding teeth exceed the anteroposterior 

 measurements throughout; the occiput greatly in- 

 creases in breadth and slowly in height; the opposite 

 grinding series become arched and strongly bent up- 

 ward, the opposite canines thus being brought near 

 together. One of the most distinctive and constant 

 features of the skull is the anteroposterior convexity 

 on top of the parietals and the general flatness of the 

 skull top in contrast to the deep concavity of this 

 region in the Menodontinae. 



4. The dental index appears to be on the average 

 somewhat lower than in the menodontine group. 

 The variability of the dental index is probably due to 

 crushing. The grinding series does not increase in 

 length so rapidly as the skull but increases greatly in 

 width. The internal cingula tend to disappear or 

 degenerate. 



5. The canines are obtuse or recurved, massive in 

 males, small in females, never elongate and pointed. 



6. The premolars are distinguished by the acceler- 

 ated development of the tetartocones, which are 

 placed farther in toward the center of the crown — 

 that is, away from the internal or lingual border, a 

 common distinctive feature; the anterior premolars, 

 p-^, are very progressive, with distinct tetartocones, 



but nevertheless are thrust inward toward the lingual 

 line and tend to drop out in old age. 



7. The internal cingula of the premolars are massive 

 and bluntly crenulate, gradually becoming more or less 

 confluent with the base of the crowns. 



8. The dominant feature of the skull is the hypertro- 

 phy or elongation of the horns and the corresponding 

 atrophy or abbreviation of the nasals, a compensa- 

 tory character. The horns are arrested in develop- 

 ment in the females, but the nasals are abbreviated 

 in both sexes, though less abbreviated in females than 

 in males. 



The brontotheriine group possessing these common 

 characters early subdivided into two very distinct 

 phyla which we term respectively the genera Mega- 

 cerops and BrontotJierium, animals which enjoyed an 

 independent simultaneous development from the 

 base to the summit of the Titanotherium zone. 

 Members of these two phyla inherited a number of 

 ancestral characters and also a number of predisposi- 

 tions to a similar evolution, which are enumerated 

 in the phyletic and family definitions above. Thus in 

 both phyla the horns progressively increase in size, 

 the teeth undergo similar changes. 



Megacerops, however, is readily distinguished from 

 Bi'ontotherium in many parts of the skull and teeth 

 and probably also in the skeleton, as we shall un- 

 doubtedly demonstrate when the skeleton becomes 

 fully known. The most conspicuous points of differ- 

 ence are shown in the accompanying table. 



Comparison of features of memhers of the Megacerops and Brontotherium pJiyla 



So far as is indicated by the breadth of the cheek 

 teeth, the curvilinear premolars, the upward flexure of 

 the premolars, and the broad zygomatic arches (for 

 example, M. hucco), Megacerops appears to be even 

 more brachyopic than Brontotherium. Megacerops 

 is less extreme in the horns than Brontotherium but 

 more extreme in the somewhat closer approximation 

 of the canines and stronger arching of the premolar 

 series. 



The grinding teeth of Megacerops are of the same 

 type as those of Brontotherium; the molars are not 

 readily distinguishable; the premolars of Megacerops 

 are smaller with reduced internal cingula. While the 

 incisors are usually absent in the adults there is 

 evidence that they were present in young animals. 

 A skull in the National Museum which resembles 

 Megacerops in its horn and nasal structure exhibits 

 large superior incisors. 



