EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 501 



Measurements of slculls and jaws associated with or referred to the Diploclonus phylum, in millimeters 



Diploclonus bicornutus (Osborn) 



{"Megacerops" bicornutus Osborn, 1902) 



Plates CIV, CVII; text figures 196, 389, 397, 409, 426, 428 



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



Geologic horizon. — Undetermined, probably middle 

 beds (Chadron B). 



Specific characters. — Skull index indeterminable from 

 type; total length, 680 millimeters; length of grinding 

 series, 340; index, 50. Nasals long and narrow (104 

 by 86 mm.). Horns subcircular (type) to transverse- 

 oval (Am. Mus. 1081, c? ) in basal section; with anterior 

 and internal hornlets; connecting crest moderately 

 developed; horns placed anteriorly to orbits. Preor- 



bital bridge of malar with a sharp edge. Occiput 

 greatly produced backward. 



This is the least specialized stage, one which is 

 closest to Brontops dispar except in the narrow form 

 of the nasals. 



Observations on the measurements of Diploclonus 

 bicornutus. — Diploclonus is a less conservative and 

 less clearly distinguished phylum than Brontops, 

 Menodus, or Brontotherium. Aside from the presence 

 of a small secondary horn swelling, a character seen 

 also in certain other skulls which are not referred to 

 Diploclonus, the type of D. bicornutus presents a 

 mingling of the characters of Allops marshi, Brontops 

 dispar, and even in some features of Menodus. In 

 comparison with these species the measurements 

 of the type skull are given below: 



Measurements of Diploclonus bicornutus, Brontops dispar, Allops marshi, and Menodus proutii, in millimeters 



pi-m' 



pi-p< 



M>-m3 . 



Pmx to condyles- 

 Zygomatic index. 



Nasal length 



Nasal breadth 



Horn length 



D. bicornutus, 



Am. Mus. 

 U76, c? (type) 



340 

 138 

 206 

 680 



104 

 183 



Nat. Mus. 

 4248, cT 



340 

 140 

 207 

 685 

 78 



94 

 102 

 188 



A. marshi, 

 Am. Mus. 



1445 

 (paratype) 



335 

 135 

 203 

 675 

 69 

 105 

 100 

 113 



Carnegie 

 Mus. 

 3063, 9 



335 

 135 

 203 

 628 



120 

 125 



150 



The table shows that these specimens, which are 

 referred to four genera, agree closely in dental meas- 

 urements but differ in the proportions of their nasals 

 and horns. 



Characters of the type. — The type of D. bicornutus 

 was long supposed to be female, but the canines and 

 zygomata are stouter than in females of other species. 

 The specific characters above set forth are based upon 

 the type skull (Am. Mus. 1476). Associated with 

 this type in the original description by Osborn as a 

 paratype was the skull Am. Mus. 1081 (PI. CV), 

 which is now regarded as more nearly related to an- 

 other species, D. tyleri. 



The following description is thus based solely 

 upon the type of D. bicornutus. The sex of this 

 specimen is somewhat uncertain, but the antero- 

 posterior diameter of the canine (506 mm.) indicates 

 that it was a male. 



The lateral compression of the type skull and jaws 

 (Am. Mus. 1476) prevents the determination of the 

 proportions of the skull or of the zygomatic index. 

 The wearing of the grinders indicates that the specimen 

 represents the eighth stage of growth. Thus we 

 conclude that all the progressive characters are very 

 distinctly developed and indicated. 



Skull. — The relatively long, narrow proportions of 

 the skull as seen from above (PI. CIV) are due partly 

 to lateral crushing. Even with allowance for some 

 deformation the skull is not very broad. As seen 

 in side view (PL CIV) it combines the long nasals of 

 A. marshi with the vertically elongate horns and 

 backward expanded occiput of D. dispar. An 

 anterior hornlet is plainly visible. The tuberosi- 

 ties on the inner sides of the horns are only faintly 

 developed, and were it not for comparison with the 

 specimens of D. tyleri they would hardly be noticed. 



