554 TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Measurements, in millimeters, of sTculls and jaws associated with and referred to members of the Brontotherium phylum 



Observations on the measurements of the Brontotherium 

 series.- — This phylum is as consecutive and as distinct 

 from other phyla as that of Menodus. The separation 

 of certain "ascending mutations" as "species" is very 

 arbitrary. Yet B. leidyi is very far removed from 

 B. platyceras. 



The generic contrasts in measurements which have 

 been given above indicate that in comparison with 

 members of the menodontine group {Brontops, Allops, 



Menodus) the male individuals of the typical spe 

 cies of Brontotherium from the upper beds are distin- 

 guished by very short tapering nasals, extremely long 

 flattened horns, long skull top, widely expanded zygo- 

 mata, very short wide premolars, and massive broad 

 molars. 



The progressive allometric evolution from Bronto- 

 therium leidyi to B. curtum and B. platyceras may be 

 epitomized as shown in the accompanying table: 



Allometric evolution from Brontotherium leidyi to B. curtum and B. platyceras 



[Measurements in millimeters] 



