546 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Seen from in front the horns are placed close together 

 at the base and diverge more at the summits than in 

 the other species, although the divergence is less than 

 in B. gigas, for example. That this divergence was 

 correlated with the development of a shallow trans- 

 verse connecting crest in the males, which was also 

 present in certain females (Am. Mus. 6349) but nearly 

 wanting in the supposed female Am. Mus. 6350 (PI. 



millimeters and are relatively much shorter than in 

 M. bucco and M. copei. This stage is therefore parallel 

 to that of B. curium among brontotheres. The 

 crania of these small animals are rather mesatice- 

 phalic than brachycephalic. The name altirostris, 

 given to the female skull by Cope, referred to the 

 elevation of the nasals above the premaxillae when 

 seen from in front. 



B 



A 



Figure 450.- 



c 



-Sections and contours of skulls of Megacerops copei and M. acer 



A, Megacerops copei, Nat. Mus, 4711 (typo) ; horns long and subvertica], their basa] section thiclc, the external face flat, and the remainder of 



the section well rounded; the section of the upper part of the horn widely oval, with a recurved external angle; no connecting crest; nasals 

 larger and broader than in ^f. acer and narrow at the end. 



B, M. acer, Am. Mus. 6348 (type); horns long and slightly inclined forward, their basal section roundly quadrate with a concave external 



anterior face; the section of the upper part of the horn transversely oval; a connecting crest; nasals short. C, M. acer, Am. Mus. 6350, 

 2 (type of Symborodon altirostris Cope, now referred to M. acer): horns relatively short and directed forward, basal section roundly 

 quadrate, with a long concave external face, section of the upper part of the horn transversely oval, nasals short and thick. 

 All one-seventh natural size. 



CL), proves that it was adapted to resist the lateral 

 strains to which the horns were subjected. Yet in M. 

 copei and M. hucco the horns are divergent and the 

 nasals are as thin between the horns as toward the tips. 

 Nasals. — Correlated with the somewhat more an- 

 terior position of the horns and partly by compen- 

 sation of growth there is a very marked abbreviation 

 of the nasals, which now measure only 60 by 114 



Occiput. — The occiput resembles M. copei also in 

 the form of the top of the occiput (fig. 451), which is 

 not expanded and, in fact, is peculiarly narrow and 

 simple. Instead of the knobs (fig. 378, F) seen in 

 Brontotherium there is a pair of pits (fig. 451) on either 

 side of the center of the occipital crest. The occiput 

 lacks the broad superior flare, which is correlated with 

 brachycephaly and the lateral motions of the skull. 



