576 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



It is probable that this skull belongs to Brontotherium 

 and not to Megacerops, because it has prevailing 

 Brontotherium characteristics. There are, however, 



Figure 476. — Horns of Brontotherium curium 

 Carnegie Mus. 560. Posterosuperior view of horns showing rugose tips 

 and accessory swellings or hornlets. (After Peterson.) One-eighth 

 natural size. 



some differences. The zygomatic buccal sec- 

 tion is somewhat more convex than that in 

 the type of B. curium, which is flatter. From 

 the evidence now at hand we might regard 

 this skull as representing an intermediate or 

 connecting stage between B. gigas and B. 

 curium, and we should expect that if com- 

 plete it would be found to possess horns and 

 nasals also intermediate between those of the 

 two species. We shall await subsequent dis- 

 coveries with great interest. 



Female sliull of B. curtum. — A small skull 

 in the American Museum (Am. Mus. 1005) 

 was formerly referred by the present author 

 to a female of B. gigas; but the nasals are 

 so excessively short and the sagittal section 

 so strongly resembles that of B. curtum that 

 this small female skull is more probably ref- 

 erable to this species. Like the female origi- 

 nally referred to B. gigas, it illustrates 

 afresh the great disparity in size between the 

 cow and bull titanotheres at this period 

 of evolution. The apparently primitive but 

 actiially sexual characters exhibited in this 

 skull have been noted alreadj^. As in the 

 female skull of B. gigas the premolar tetarto- 

 cones are less distinctly circular than in the 

 males. The internal cingulum is less reduced, 

 the occiput is not so greatly prolonged back 

 of the zygoma, and the canines are short 

 and recurved rather than bulbous. This list 

 of sexual characters appears like a summary 

 of primitive characters. The premaxillaries 

 are edentulous, or toothless, confirming the 

 evidence afforded by the B. gigas female skulls, 

 that in the brontotheres the upper incisors were re- 

 duced or wanting in the females — a conclusion, how- 

 ever, which requires final substantiation by additional 



evidence. The malar bridge over the infraorbital 

 foramen is broader than in the male. While the 

 frontal section closely resembles that of B. curium, 

 the sexual disparity is illustrated not only by the light 

 zygomata and narrow occiput, but by the irregularly 

 osseous summits and by the form of the horns, which 

 are less flattened posteriorly than in the male type of 

 this species (Pis. CXC, B; CXCII; figs. 459, B; 477, B). 

 Of interest is the vertical septum extending from 

 the under surface of the nasals to meet a similar 

 septum rising from the premaxillaries, which is seen 

 also in other skulls in the different phyla. WhUe in- 

 complete, this septum illustrates a tendency toward 

 the formation of an intranarial septum similar to that 

 in the rhinoceroses. 



A 



B 



Figure 477. — Sections and contours of skull of Brontotherium curtum 

 A, British Mus. 5629. One-eighth natural size. This specimen has short horns, a high con- 

 necting crest, and moderately expanded zygomata, as in the supposed females of Bronto- 

 therium, but the canines are larger than in females and suggest that this is a male. It 

 resembles Cope's type of " Menodus peltoceras." The basal section of the horns is flat- 

 tened to concave posteriorly and has a flattened external face. The nasals are small and 

 pointed. B, Am. Mus. 1005,9. The horns are even shorter than in the preceding speci- 

 men, and the connecting crest is equally high. The basal section of the horns is wide, 

 flattened posteriorly and antero-externally; the zygomata are moderately expanded. 



Another character which parallels especially the 

 brachycephalic rhinoceros is the flattening out of the 

 lower lateral portion of the occiput. 



