574 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



premolars and molars and for the shortness of the 

 nasals. The other two skulls provisionally entered 

 under this species above (p. 553) have much smaller 

 dental measurements, about as in B. gigas or Jiatcheri, 

 and may belong in either species. The female skull in 

 the British Museum, referred provisionally to B. pel- 

 toceras, approaches the female skull referred to B. 

 curtum (Am. Mus. 1005) except that the nasals are 

 smaller. Future investigation may settle whether 

 B. medium is distinct from B. gigas elatum. 



very distinct tetartocones ; external cingula feeble; 

 internal cingula reduced. Nasals greatly abbrevi- 

 ated, free length 52 millimeters. Horns greatly 

 elongated in males (355-380 mm.), in females abbrevi- 

 ate (160 mm.); basal section of horns strongly con- 

 vex anteroposteriorly, slightly convex to plane pos- 

 terioi'ly, breadth moderate. Zygomata with buccal 

 expansions broad, plane above, bulging at the sides, 

 plane below. Skull mesaticephalic, basal length cf 

 790-840 millimeters, zygomatic breadth c? 620 milli- 



A B C 



Figure 475. — Sections and contours of skulls of Broniotherium medium and B. curtum 



A, Brmtoiliermm medium, Nat. Mus. 4256 (type), short nasals approaching in form those of B. curium, but horns not so long; basal section of horns has a 

 flattened external face. B, B. curtum, Nat. Mus. 4946. C, B. curtum, Yale Mus. 12013 (type). In B. curtum nasals are very short, horns long, basal 

 section well rounded externally and flat posteriorly, zygomata well expanded. One-ninth natural size. 



Brontotherlum curtum (Marsh) 



{Titanops curtus Marsh, 1887; fMenodus peltoceras Cope, 1891; 



" Brontotherlum curtum" Osborn, 1902) 

 Plates XX, XLVII, CLVII, CLXXVII-CLXXX, CLXXXIII- 

 CLXXXVI, CXC, CXCII-CXCIV; text figures 182, 390, 

 392-394, 399, 457-459, 475-478, 709, 719, 744 



[For original description and type references see p. 224] 



Geologic horizon. — Upper Titanoiherium zone of 

 South Dakota. 



Specific and generic characters. — Molar-premolar 

 series 350 millimeters (estimated). Premolars with 



meters; zygomatic index 73-77. Vertex of skull 

 elongated by expansion of occiput back of the zygo- 

 matic arches and by forward shifting of the horns. 

 Female skull much smaller (nasals to top of occiput 

 635 mm.). 



The chief advances in this stage beyond B. gigas 

 and B. medium are in the still further abbreviation 

 of the nasals, the more flattened posterior face of the 

 horns, the more backward extension of the occiput, 

 all progressive characters which are bridged over more 

 or less fully by transitional types. The disparity 



