EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



577 



Additional observations on the supposed female oj 

 Brontotherium curium. — The female skull described 

 above (Am. Mus. 1005) has been referred to this 

 species because of the abbreviated nasals, high 

 connecting crest, and flattened horns. The shortness 

 of the horns has been assumed to be a female char- 

 acter. The systematic position of this skull, however, 

 is hardly free from doubt, and the remarks made above 

 with reference to the supposed female of B. gigas may 

 possibly apply here, especially in view of the dimen- 

 sions of p* (ap. 40 mm., tr. 61) and m^ (84 by 82), 

 which suggests reference to the Menodontinae rather 

 than to the Brontotheriinae. However, the premolar 

 external cingula in the present specimen are obsolete, 

 the tetartocones are better developed, and the antero- 

 posterior measurements of the dentition are also close 

 to those of male skulls of B. curtum, with which species 

 this skull may be left for the present. 



Female sTcuU, Cope's type oj Menodus peltoceras 

 (Am. Mus. 10719). — This type, described in detail 

 in Chapter III, page 230, confirms the evidence afforded 

 by the American Museum specimen No. 1005 as to 

 the shortness of the horns in the females of these 

 upper-level brontotheres. It resembles Am. Mus. 

 1005 in the extreme abbreviation of the nasals, in 

 the steepness and size of the connecting crest, and 

 in the marked prominence of the vertical ridge on 

 the outer side of the horn. In sagittal and basal 

 section the specimen agrees best with the female skull 

 referred to B. curtum, but the still more extreme 

 reduction of the nasals is equaled only in the species 

 B. platyceras, to which this fragment may possibly 

 belong. 



Conclusion. — The type specimen of M. peltoceras 

 probably represents a female of one of the long- 

 horned species of brontotheres, probably B. curtum. 



A finely preserved skull in the British Museum, 

 (No. 5629) is represented in Plates CXCIII, CXCIV 

 and Figure 477, A. 



It presents a puzzling character in the large size 

 of the canines, which resemble those of a male; in 

 all other measurements it agrees with the male speci- 

 mens of B. hatcheri and B. gigas, but in the con- 

 formation of the peculiar shield in front of the skull 

 and of the abbreviated nasals this certainly resem- 

 bles the supposed female of B. curtum (Am. Mus. 

 1005). It also exhibits a still stronger resemblance 

 to the type of Menodus peltoceras Cope just described. 

 The canines and grinding teeth in form are those of 

 a true Brontotherium. It is difficult to interpret this 

 specimen satisfactorily. If it is actually a male it 

 may indicate that B. peltoceras was a distinct, peculiar 

 species of bronto there with short horns; or this may 

 be an aberrant specimen, either a female in which 

 the canines are exceptionally large, or an aberrant 

 male in which the horns are exceptionally short. 



This specimen certainly raises some doubt as to 

 our interpretation of the female sex of the skulls 

 described above. 



Brontotherium ramosum (Osborn) 



{Titanolherium ramosum Osborn, 1896; "Brontotherium ramo- 

 sum" Osborn, 1902) 



Plate CXCI; text figures 194, 457, 479 

 [For original description and type references see p. 231] 



Geologic horizon. — Upper Titanolherium zone of 

 South Dakota. 



Specific characters. — I^-, C^, P^, M^. Premolars 

 with obsolete external and reduced internal cingula; 

 with two distinct internal cones. Molar-premolar 

 series 350 millimeters. Skull brachycephalic. Horns 

 elongate, in males 399 millimeters; gently plano- 

 convex at the base. Very broad, gently biconvex and 

 laterally expanded at the summits, with a very deep 

 connecting crest. Nasals greatly abbreviated. Zygo- 

 mata expanded into two wide, flat plates. 



Figure 478. — Left horn and nasals of Brontothe- 

 rium curtum? (female) 



Am. Mus. 10719 (type of "Menodus peltoceras"); White River, 

 Colo. Front view. One-fourth natural size. This fragment 

 represents the left horn and coossified nasals of a brontothere 

 allied in form to British Mus. 5629. It may have been a 

 female of one of the long-horned, short-nosed types {B. 

 curtum, etc.). 



This species or ascending mutation appears to be 

 a further evolution of the B. gigas, B. curtum, B. 

 medium phylum. It is related in many characters 

 to B. curtum and, on the other hand, is transitional 

 toward B. platyceras, as shown in the sections of the 

 horns, nasals, and zygomatic arches (fig. 479). 



This "branching" or " spreading-horned " bronto- 

 there is represented only by the type skull of a very 

 old bull in the American Museum collection (No. 1447) 

 in the tenth stage of growth, and by a pair of horns 

 with connecting crest in the National Museum 

 (No. 1243). 



From this somewhat scanty material the validity 

 of the stage entitled B. ramosum awaits confirmation. 

 As a proof of extreme age even the cingulate hypocone 

 of m' is well worn, a rare occurrence. Note also that 



