568 



TITANOTHERES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



inches (90 mm.), and the zygomatic index rising to 

 89 as compared with 62 in a typical Menodus giganteus. 

 The proportions of the huge skeleton belonging to the 

 same large bull (Am. Mus. 492) show corresponding in- 

 crease in the length of the ribs, height of the dorsal 

 hump, and width of the pelvis (see above). 



Materials. — Our knowledge of this species is de- 

 rived from exceptionally rich materials, including es- 

 pecially the type jaw of B. gigas, the splendid type 

 skull and jaw of B. {Titanops) elatum in the Yale 

 Museum (No. 12061), and the superb skull of B. gigas 

 in the American Museum (No. 492), broadly recorded 

 from the upper Titanotherium zone. Associated with 



(allowing for a slight difference of age), in form of jaw, 

 canines, and molars (especially ms), in the ratio of 

 premolars to molars. It differs only in the presence of 

 Pi, which has dropped out in B. gigas, and in the 

 reduction of the cingulum on the premolars. The 

 relation of these species is further confirmed by speci- 

 mens in the National and American Museums which 



A 



B 



Figure 470. — Sections and contours of skulls of Bi-ontotherium hatcheri and B. gigas 



A, BrontotJierium hatcheri, Am. Mus. 1070; borns shorter than in B. gigas and connecting crest lower; hasal section of horns'roundly 

 trihedral, not so wide as in B. gigas. B, B. gigas, Yale Mus. 12061 (type of Titanops elatus)', horns long, erect, and placed 

 far in front of the orbits, their basal section roundly trihedral, section of the upper part of the horn a transversely extended 

 oval, nasals fairly long, parietal crest wide. C, B. gigas. Am. Mus. 492; horns stouter, basal section more rounded oval and 

 upper section less flattened, nasals shorter, and zygomata widely expanded. One-ninth natural size. 



the skull of B. gigas are a pelvis, forearm, manus, and 

 dorsal vertebra with ribs. There are also two skulls 

 in the National Museum — No. 4262, from the lower 

 levels of the upper Titanotherium zone, a young male 

 in the fifth stage of growth, associated with consider- 

 able portions of the skeleton, and No. 4244, with 

 imperfectly preserved horns, recorded from the top 

 levels of the upper zone. There is also the skull of a 

 small but aged female (Am. Mus. 1006). 



Relation of Brontotherium gigas and Titanops ela- 

 tus. — A comparison of the type jaw of B. gigas with 

 that of Titanops elatus shows that the former repre- 

 sents a younger, the latter a somewhat older and more 

 robust individual of the same species; the type T. 

 elatus jaw (see description below and measurements 

 above) resembles the type B. gigas jaw in size 



are described below. The additional generic char- 

 acters of Brontotlierium must, therefore, be derived 

 not from the skull which Marsh referred to B. ingens, 

 but from the skulls which he referred to Titanops 

 elatus and Titanops curtus. 



Our conclusion is that the type skull and jaws of 

 B. (Titanops) elatum belong to a somewhat larger 

 male animal than the type jaw of B. gigas and may 

 represent a more progressive ascending mutation, 

 which might be termed B. gigas elatum. The skull 

 and jaws of B. (Titanops) elatum are certainly more 

 progressive than those of B. liatcheri. 



Ohservations on the type jaw of Brontotherium gigas. — 

 The type lower jaw was figured by Marsh (1876.1, pi. 

 12) and is well represented in Plate CLXXI of this 

 monograph. 



