824 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



13. Foramen ovale approaching foramen lacerum medium. 



14. Postcanine diastemata disappearing. 



15. JMandibular rami develop a short, triangular coronoid 

 process, condyle elevated, mandible with depressed angle; 

 face of coronoid flattened in front, tending to overhang third 

 inferior molar; rami increasing in depth and thickness; diastema 

 between canines and first premolars disappearing. 



attributed to Palaeosyops are the expressions of its 

 progressive brachycephaly. These so-called generic 

 characters are based upon the harmonic trend, or 

 direction, of evolution in the skull as a whole with 

 its correlated changes in a large number of biochar- 

 acters. Some of the "specific" characters which the 



Figure 741. — Skulls of three species (probably successive) of Palaeosyops from the middle Eocene, Bridger 



formation 

 A, P. major, level B 3; B, P. Icidyi, level probably upper C or D; C, P. robustus, level probably D. 



16. Incisors become more transverse in position; canines 

 stouter and rounder. 



17. Molars become broader than long; parastyle robust; 

 robust internal and external cingula. 



18. Hypoconulid of ma from a subcrescentic form acquires a 

 progressively conic form. 



HARMONIC PROPORTION TREND CONSTITUTES THE 

 GENERIC CHARACTER OF PALAEOSYOPS 



The chief "generic" characters which Leidy, Cope, 

 Earle, Osborn, and others have from time to time 



same authors have attributed to Palaeosyops are the 

 successive stages in its progressive brachycephaly. 

 Other specific characters consist of successive rectigra- 

 dations which are added to the teeth and the skull. 



FOURFOLD MODES OF ALLOMETRIC BRACHYCEPHALY 



From comparison with the skulls of other mammals 

 we draw four important conclusions in the study of the 

 allometric brachycephaly of the skull of Palaeosyops 

 as a whole, namely: 



