EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



493 



Draw," near the base of Chadron C (upper Titanothe- 

 rium zone), 60 feet below the summit, as recorded by 

 Hatcher. This skeleton is so complete that almost all 

 its parts were figured for this monograph in plates and 

 woodcuts under the direction of Professor Marsh. 

 Brontops rohustus appears to have been the predomi- 

 nant animal during the deposition of the beds of the 

 lower C 1 levels of the upper Titanotherium zone in 

 South Dakota. It is a comparatively short-horned 

 type, contemporaneous Vv'ith the early members of the 



that raises a doubt as to the invariable persistence of 

 the median incisor. These and other characters relate 

 this animal to Brontops hrachycepJialus and B. dispar, 

 in which the median superior incisor is also variable. 



Convergence of Brontops rohustus to BrontotTierium. — ■ 

 The typical B. rohustus is very easily distinguished, 

 but there are at least six other skulls that simulate 

 Brontotherium in the premolars, in the expansion of 

 the zygomata, and in the flattening of the horns. In 

 some of these also the skull vertex appears long, but 



B C 



Figure 421. — Sections and contours of skull of Brontops rohustus 

 Nat. Mus. 4696; middle levels of Chadron C; either a very large and progressive B. dispar or a primitive B. robustus, with stout cylindrical horns far 

 in front of the orbits, broadlv and roundly trihedral in section. Nasals short and vride. B, Am. Mus. 1083, referred to B. rohustus, although the horns 

 are roundly trihedral in section. C, Yale Mus. 12018 (type); lower levels of Chadron C; very thick horns incUned forward and far in front of the 

 orbits, in basal section very widely oval; nasals very short and thick, zygomata massive, and parietal crest wide. All one-eighth natural size. 



long-horned series of brontotheres, such as B. gigas. 

 The long axis of the basal horn section tends to become 

 directly transverse, and the posterior face is somewhat 

 flattened; the nasals are still broad, resembling closely 

 those in some specimens of B. dispar. The presence 

 of two large upper incisors in the type skull of this 

 species was believed by Marsh to be its most distinc- 

 tive character; two upper incisors are observed also 

 in skulls of the same species in the Yale and American 

 Museums. The male skull (Princeton Mus. 11015), 

 however, carries but one lateral superior incisor, a fact 



it never has the midparietal protuberance, and the 

 nasals and horns are nearer to B. rohustus than to 

 Brontotherium; also the internal cingula of the pre- 

 molars are well rounded, and the canines are pointed 

 at the tips. 



Observations on the measurements of Brontops ro- 

 hustus. — The males of this species exhibit a con- 

 vergent resemblance to Brontotherium in the buccal 

 expansion of the zygomata, in the reduction of the 

 external cingula of the premolars, in the progressive 

 well-rounded premolar tetartocones of certain speci- 



