494 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



mens, and in the massive form of the canine. The 

 internal cingula of the premolars, however, are pro- 

 nounced and well rounded as in Brontops. The refer- 

 ence of these specimens to Brontops roiustus rather 

 than to Brontotherium is confirmed by the following 

 contrasting measurements : 



Measurements of Brontops, Brontotherium gigas, and Bronto- 

 therium platyceras, in millimeters 



In general Brontops rohustus is distinguished from 

 Brontotherium by the greater length of the premolar 

 series, by the shorter skull base and skull top, and by 

 the larger canines, which are also more pointed at 

 the tip. The horns are much shorter than in the flat- 

 horned species of BrontotTierium, and the connecting 

 crest is lower. 



The range of size in skulls referred to this species is 

 indicated as follows: 



Millimeters 



SkuU, basilar length 743-813 



Pi-m3 340-376 



Pi-p^ 132-151 



M'-m" 230-237 



Zygomatic index 77- 87 



Outside length of horns 130-210 



The premolars and molars are wide compared with' 

 those of Menodus: 



Brontops robustus 



Menodus trigonoceras. 



40X65 to 46X69 

 43X51 to 41X55 



73X84 to 90X89 

 72X70 to 82X79 



Geologic and geograpJiic distribution. — So far as re- 

 corded, all the known specimens of this species were 

 obtained from the Titanotherium zone (Chadron for- 

 mation) of South Dakota. It appears probable that 

 the geologic level recorded for the type specimens by 

 Hatcher, namely, lower C of the upper Titanotherium 

 zone, is characteristic. 



Materials. — The type skull and skeleton in the Yale 

 Museum (No. 12048) afford the best knowledge of the 

 extreme characteristics of this type. In the American 

 Museum there are two fine skulls (Nos. 1083, 1069) 



which confirm or supplement the characters observed 

 in the type. In the Princeton Museum there are two 

 male skulls, also found in "Corral Draw," South 

 Dakota (Nos. 11439, 11015), associated withmany 

 parts of the skeleton; also a valuable skull (No. 10061) 

 which is transitional between B. robustus and Allops 

 marshi in the disposition of the horns. In the Na- 

 tional Museum there is an extremely brachycephalic, 

 robust skull (Nat. Mus. 4253) of smaller size than the 

 typical B. robustus, which may represent a primitive 

 or transitional variety of this species. It agrees in all 

 its principal characters with the type skull, although 

 smaller and in many features more primitive. 



Slcull. — The most prominent cranial characters of 

 this well-defined species are extreme brachycephaly, 

 extreme abbreviation of the occiput, marked depres- 

 sion of the midregion of the cranial vertex, rapid 

 elevation of the summit of the occiput, downward and 

 forward inclination of the zygomata. In female 

 specimens of B. dispar the length exceeds the breadth 

 by 200 millimeters; in the type of B. validus the length 

 exceeds the breadth by 100 millimeters. In this skull 

 also the length exceeds the breadth by only 100 

 millimeters. This feature stands in widest contrast 

 with the dolichocephaly of the contemporary Menodus 

 giganteus, in which the length exceeds the breadth by 

 over 230 millimeters. The type skull, with its 

 abbreviate occipital region, also contrasts with that 

 of the contemporary Brontotherium gigas, which shows 

 a decided backward extension of the occiput, especially 

 in the males. The age or growth characters of this 

 skull are similar to those in some of the collateral 

 ancestors of this phylum — namely, expansion and not 

 very marked elongation of the horns, which shift 

 progressively forward and reach an extreme over- 

 hanging position in the type. Old skulls exhibit a 

 thickening of the connecting crests between the horns, 

 also the distal growth, widening and rugosity of the 

 nasals, and thickening of the buccal swellings of the 

 zygomata. These buccal swellings are somewhat 

 crushed laterally in the Yale type specimen ; but in all 

 the skulls examined the buccal section appears to be 

 smaller and more concave above, and on the outer 

 border a less strong flaring out at the sides is observed 

 than in the skulls of Brontotherium medium or B. 

 curtum. 



In general, the skull apart from its much greater 

 brachycephaly resembles on a grander scale that of 

 Allops marshi, both in its superior and inferior aspects 

 and in the form of the nasals, yet the retarded condi- 

 tion of the premolar tetartocones, the extremely wide 

 transverse sections of the horns, and, the relative 

 obtuseness of the canines do not point to direct rela- 

 tionship to A. marshi. 



Horns. — Horns are found, however (as in Am. Mus. 

 1083, 1069, Princeton Mus. 10061), which are transi- 

 tional in position and basal section between those of 

 Allops marshi and of this species; progressive varieties 



