32 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



one part is taken up by some other part. For exam- 

 ple, the loss of the function of the incisors in the pre- 

 hension of food is compensated for by changes in the 

 form and function of the lips. 



upper lip like that of the tapir necessitates space for 

 the superior retractor muscles, which curl the lip 

 upward and backward. An example of the results of 

 the evoUition of the lower jaw may be seen by compar- 



FiGtTRE 24. — Restorations to the same scale of the heads of some of the principal t3'pes of titanotheres 



Drawn by Charles E. Knight, after models made by him under the author's direction. About one-seyenteenth natural size. A, 

 Brontops roiustus Marsh, oblique yiew, middle Titanotherium zone; B, Menodus giganleus, upper Titanotherium zone; C,Megacerops 

 copei Cope, partly oblique side yiew, summit of the Titanotherium zone of Colorado; D, Broniotherium platyceras Scott and Osborn, 

 the final stage in the eyolution of the horns of the titanotheres, summit of the Titanotherium zone of South Dakota; E, Protitano- 

 iherium sp., summit of the Eocene. 



With the evolution of the lips the structure of the 

 anterior parts of both the upper and lower jaws, of 

 the anterior teeth, and the anterior nasal openings is 

 closely correlated. The development of a prehensile 



ing Eotitanops gregoryi and Brontotherium {medium) 

 gigas, the whole jaw of the former hardly exceeding in 

 length a single posterior grinding tooth of the latter. 

 (See fig. 25.) 



