EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



551 



flattened oval section of the horns, the progressive 

 connectuig crests between the horns, and ia general 

 by their superior if not dominant size. The jaw of 

 B. gigas (PL CLXXI) is very characteristic of the 

 genus, with its recm-ved coronoid process, shallow 

 chin, robust mandibular section, two pairs of incisor 

 teeth, and obtuse rounded canines. The finely pre- 



Horns. — The males of all these animals exhibit 

 progressively broad and flattened horns, transversely 

 oval from base to summit, diverging widely at the 

 summits, with the great connecting crest rising be- 

 tween them (unlike the typical Megacerops), to resist 

 lateral strains, and evolving at the expense of the 

 constantly diminishing free portion of the nasals 

 (fig. 457). 



Skull. — The top view of the cranium is very long 

 (fig. 459), owing to the extreme forward position of 

 the horns and the great backward extension of the 

 occiput behind the zygomatic arches. The cranium 

 proper is dolichocephalic; the expansion of the buccal 

 processes of these arches is, however, so broad that 

 the total breadth of the skull finally equals the total 



Figure 457. — Composite sections showing tlie evolution of the horns and reduction of the free nasals in the 



Brontotherium phylum 



a, Broniothenum leidyi, Nat. Mus. 4249 (type), Chadron A 2; &, Brontoihenum hypoceras, Nat. Mus. 4273, Chadron A 2; c, BTonlotherium hypoceras, 

 Nat. Mus. 4702, Chadron A 37; d, Brontotherium hatcheri. Am. Mus. 1070, Chadron; e, Brontotherium gigas, Yale Mus. 12061 (type of B. elatm), 

 upper (?) Chadron; f, Brontotherium gigas, Am. Mus. 492, Chadron C; g, Brontotherium curtum, Yale Mus. 12013 (type), Chadron C; h, Brontothe- 

 rium curtum, Nat. Mus. 4946, Chadron C 3; i, Brontotherium ramosum. Am. Mus. 1447 (type), Chadron C; j, Brontotherium platyceras. Am. Mus. 

 1448, Chadron C. All one-fourth natural size. In the earliest stage, B. leidyi, the horns are small and placed near the orbits, there is no connecting 

 crest, and the nasals are long and slender; in the latest and most specialized stage, B. platyceras, the horns are extremely long and placed far in 

 front of the orbits, the coimecting crest is very high, and the free portion of the nasals is practically vestigial. Between these extremes lie a con- 

 siderable number of intermediate stages. 



served type skull of B. (Titanops) elatus (PL CLXXV) 

 enables us fully to characterize the genus. The suc- 

 ceeding and stni more advanced stages in the evolu- 

 tion of this phylum were originally named as foUows: 

 Menodus dolichoceras Scott and Osborn, Titanops 

 curtus Marsh, Titanops medius Marsh, TitanotJierium 

 ramosum Osborn. 



101959— 29— VOL 1 38 



length, the zygomatic index in B. platyceras being 

 110, or hyperbrachycephalic. This condition is best 

 imderstood when the skuU is viewed from below 

 (fig. 393). The whole structure of the skull, including 

 the broad and spreading occiput (fig. 378) and the 

 excessive buccal expansion, is adaptively adjusted to 

 the development of the horns, which from their feeble 



