446 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



male skull of M. giganteus (Am. Mus. 505, 1066, 1067; 

 fig. 444) has stout triangular horns, whereas the female 

 skull (Am. Mus. 506; fig. 445), although a very large 

 specimen, has more slender horns, which are rounded 

 in section, and has very slender canines. Similar 

 sexual differences are observed in specimens of M. 

 trigonoceras. 



Sport variations in the horns. — It is difficult to deter- 

 mine whether the internal "hornlet," or branch horn, 

 which appears on the inner side of the main horn in 

 specimens belonging to at least three different phyla 

 (Diploclonus, Menodus, Brontotherium) is a veritable 

 progressive character — that is, a rectigradation — or 

 merely a sport. This branching horn is believed to 

 be a generic character of the animal called Diploclonus 

 amplus by Marsh. Another example of internal 



Figure 377. — Male and female skulls of Brontotherium 



gigas 

 A, Am. Mus. 1006, 9 ; B, Am. Mus. 492, <? . One-twelfth natural size. 



branching is that shown by a female of Brontotherium 

 gigas elatum (Am. Mus. 1006). 



Effects of geologic crushing. — Vertical or lateral 

 crushing not only profoundly influences the entire 

 proportions of the skull but totally alters the shape 

 and angulation of the horns, as illustrated in two 

 examples of Menodus trigonoceras and Megacerops 

 copei (figs. 439, 394, G), one of which is crushed ver- 

 tically, the other horizontally. 



NASALS; EXPANSION, ABBREVIATION 



The hypertrophy of the horns and the compensating 

 atrophy of the nasals were pointed out by the writer 

 in 1887. Primitive nasals are invariably elongate, but 

 they show a fundamentally characteristic form, which 

 again distinguishes two groups. In the menodontine 

 group they are distally broad and truncate, as in 

 Menodus, primitively narrow and broadening distally, 



as in Brontops, and moderately broad and laterally 

 decurved, as in Diploclonus; in the brontotheriine 

 group they are distally tapering and decurved, as in 

 Megacerops, and progressively abbreviate and pointed, 

 as in Megacerops and Brontotherium. 



The shape of the nasals, however, does not sharply 

 distinguish all members of the two groups, as there is 

 more or less convergence between the members of 

 different phyla. In members of both groups the 

 tendency to shorten the nasals appears to be progres- 

 sive; it is less marked in Menodus and is carried to the 

 greatest extreme in Brontotherium. 



In the Menodus group an age character is the distal 

 broadening, expansion of the nasals, as seen in the 

 comparison of young and old specimens of different 

 species of Brontops. 



ZYGOMATIC ARCHES: EXPANSION, BUCCAL PLATES 



Expansion oj the arches. — The progressive spreading 

 of the posterior portion of the zygomatic arches 

 (figs. 391-393) is a highly characteristic feature of 

 members of both the menodontine and brontotheriine 

 groups. The more primitive titanotheres in both 

 groups exhibit moderately expanded zygomata with- 

 out any rugose areas. The rugose development of 

 the zygomatic arch takes place pari passu with the 

 massive development of the horns. Thus the highest 

 degree of zygomatic expansion characterizes the great 

 brontotheres (figs. 392, 394) in which the horns reach 

 their maximum development. This becomes a sexual 

 character; the extremely robust and widely spreading 

 zygomatic arches of the more progressive species of 

 males present a contrast with the moderately expanded 

 arches that are associated with the feeble or imperfect 

 horns and small canines of the less progressive females. 

 In males and females of Menodus the same differences 

 are observed, but in a less marked degree than in 

 Brontotherium. The less expanded zygomata of the 

 female skulls give them a less brachycephalic and 

 more primitive character throughout the phylum. 

 Thus in the nasals, in the horns, and in the zygo- 

 matic arches the males always appear more progressive 

 and the females more primitive. 



Zygomatic cephalic indices. — The expansion of the 

 zygomatic arches is so much more rapid than the 

 elongation of the skull as a whole that the breadth 

 across the zygomata nearly if not quite equals the 

 basilar length; thus a skidl which is really elongated, 

 like that of Brontotherium platyceras, presents a high 

 zygomatic index, whereas the brachycephalic general 

 character is less marked in the grinding teeth and in 

 the parietal vertex of the skull (compare fig. 390, B, 

 D,F). 



OCCIPITAL PILLARS; AUDITORY MEATUS 



The occiput. — Correlated with the progressive evo- 

 lution of the horns is the progressive transformation 

 of the occiput from the transversely convex contour 



