EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHEEES 



581 



Measurements of Brontotherium curtum and B. platyceras, in 

 millimeters 



P'-m^ 



Pi-p* 



M'-m^ 



Pmx to condyles- 

 Zygomatic index - 



Nasal length 



Horn length 



B. curium B. platyceras 



345-350 

 128-130 

 218-228 

 780-840 

 74-?78 

 52-65 

 355-380 



337-340 



120-123 



221-223 



728-880 



80 



20-38 



390 



Thus B. curtum is a little larger than B. platyceras 

 in the dental measurements but has longer nasals and 

 shorter horns. 



In B. platyceras the type consists only of a pair of 

 horns and hence affords no dental measurements. 



The first referred skull (Am. Mus. 1448) is crushed, 

 so that the length of the horns has probably been in- 

 creased, the basilar length perhaps lessened. The 

 true molars are not as large as in certain specimens of 

 B. gigas, but the ratio of molar length to the basal 

 length of the skull is at least not less than in other 

 brontotheres. In basilar length the first specimen of 

 platyceras is smaller than curtum, the second speci- 

 men is much larger. 



The second referred skull in the Field Museum of 

 Chicago (No. 12161) is the largest titanothere on 

 record and is superbly preserved (Pis. CLXXXI, 

 CLXXXII). Its flattened spreading horns combine 

 features of B. platyceras and B. ramosum. Its molar 

 index (25) is lower than in typical brontotheres 

 (28-29). The premolar series is short. 



