EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



483 



plication, as well as from those of Brontoiherium, 

 which are rapidly progressive. The presence of both 

 internal and external cingula is a character which 

 Brontops presents in common with Menodus. 



Skull. — It is to be noted that the horns shift forward 

 and evolve at the expense of the nasals, as in Mega- 

 cerops and Brontoiherium, whereas in Menodus the 

 horns are stationary in their relative position. The 

 progressive brachycephaly of Brontops affects chiefly 



Specific cTiaracters. — Skull small, premaxillaries to 

 condyles about 580 to 590 millimeters; males with 

 strongly expanded zygomata, zygomatic index 82; 

 transverse measurement about 480 millimeters. 

 Females more mesaticephalic, zygomatic index 64. 

 Horns very short ; outside length in transitional skulls 

 (male), 85 to 135 millimeters; horn section antero- 

 posteriorly elongate to roundly trihedral. Nasals 

 elongate, not spreading distally; free length (esti- 



B 



mm, 



A 



m 



D 



-Sections and contours of skulls of Brontops brachycephalus 



Figure 414.- 



A, Nat. Mus. 4947, 9 (cotype) and B, Nat. Mus. 4261 (type) ; botb from Chadron A. In these more primitive members the horns are small bosses, rounded to elongate oval 

 in basal section, the nasals are long and tapering, the parietal crest is narrow. C, Nat. Mas. 1214; upper levels of Chadron A; horns distinctly larger, rounded trihedral 

 in section with external, internal, and posterior faces; nasals thicker and relatively shorter. D, Nat. Mus. 4268; base of Chadron B; an immature animal of more pro- 

 gressive type with horns larger and oblique oval in section, nasals wider, zygomata stouter. All one-seventh natural size. (Of. flg. 376.) 



not only the zygomatic arches, which spring strongly 

 outward, especially in the old males, but also the entire 

 vertex of the skull. 



Brontops brachycephalus (Osborn) 



{Megacerops brachycephalus Osborn, 1902) 



Plates XXI, XLVII, LXXXIII-LXXXVIII; text figures 195, 



374, 387, 389, 391, 393, 396, 399, 406, 408-419, 610, 611, 712 



719, 744 

 [For original description and type references see p. 231. For skeletal characters 

 see p. 676] 



Geologic horizon. — Lower Titanotherium zone (Chad- 

 ron A, A 1, A 2, A 3). 



mated), 85 to 109 millimeters. Nasal index, 90 to 

 140. Grinding series, p'-m', in males 280 millimeters, 

 in females 253 ; dental index in males 48, in females 43 ; 

 true molars, in males' 178 millimeters, in females 150. 

 Canines not strongly compressed anteroposteriorly, 

 length of crown in males 34 to 40 millimeters, in 

 females 34. Tetartocone of p* rudimentary (a con- 

 cave spur from the deuterocone continuous with the 

 internal cingulum)^ tetartocones of p^ and p' small, 

 constriction separating tetartocone from deuterocone 

 in the hinder third of the crown. Backward pro- 

 longation of occiput moderate. 



