552 



TITANOTHERBS OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



development in the females (as in the type of Menodus 

 peltoceras Cope {=lBrontoiherium curtum, figs. 459, 

 477, 478) are judged to have been fighting weapons 

 in the males. 



FHarv.Mu5..type 



EYaleMu5.l20l3,type 



D Yale Mu5. 12061 



CAM. 1070 



B Nat.Mus.427Jn«)type 



ANat.Mu5.4249,t^pe I -^ ^y nhypoceras AZJi.n^^otyf^ 



lleidyi N.M. 4249, type 



Figure 458. — Basal sections of the horns in the Bi-ontotherium phylum 



A, Brmlotherium Icidyi, Chadron A 2; B, £. hypoceras, Chadron A 2; C, B. Jiatcheri, Chadron; D, B. gigas, Chadron 

 ?C; E, B. curtum, Chadron C; F, B. platyceras, Chadron. These sections show the progressive change from 

 the small anteroposterior oval of B. Uidyi to the very wide transverse oval of B. platyceras. 



increase in size of the skull as a whole (24 per cent). 

 The total length of the premolar series diminished, 

 but in B. gigas the individual molars are enormous — 

 for example, m^ (ap. by tr.) 91 by 94 millimeters, as 



Figure 460.- — Contrast in contour of horns and 

 nasals between male and female brontotheres 



The female (a), Am. Mus. 1006 (referred to B. gigas), differs 

 from the male (b) , Am. Mus. 492 (referred to B. gigas), 

 chiefly in having short horns and nasals, combined with a 

 high connecting crest. 



Figure 459. — Skulls cf male and female brontotheres 

 Top view. About one-ninth natural size. A, Broniotherium platyceras. Am. Mus. 1448 (cotype) . In this male skull vertical crushing has 

 emphasized the width of the zygomata. B, B. curtum (peltoceras). Am. Mus. 1006. This female skull is small, the horns are short, 

 and the zygomata are unexpanded. 



Grinding teeth. — The grinding teeth suffered prob- 

 ably from the highly "selective value" placed upon 

 the horns and were retrogressive in many respects; 

 the total increase of length in the grinding series 

 (21 per cent) thus did not fully share in the progressive 



compared with M. giganteus (Am. Mus. 505), m^ 87 

 by 84. The breadth of p* was also exceptionally great, 

 and nowhere else do the tetartocones attain such 

 vigorous development as in B. gigas, not excepting 

 Menodus giganteus. 



