TERTIARY MAMMAL HORIZONS. 7 



premolar of the Plagiaulacida; or of the premolars of the Pcris- 

 sodactyla afford certain veiy exact data for correlation purposes. 



3. Simultaneous Ixtroductiox of New Forms: the sud- 

 den appearances in both Nearctic and Palaearctic regions of 

 t\'pes which have no known ancestors in lower horizons and 

 have apparently originated elsewhere, either in Africa or in South 

 America. The value in chronology of these immigrations has 

 not been fully recognized. 



4. Predominance of Certain Types : the predominance or 

 spread of certain types as of the Perissodactyla in the Middle 

 Eocene, or of the Artiodactyla in the Upper Eocene and Oligo- 

 cene are in the nature of supplementar}^ evidence. 



5. Convergence and Divergence of Pal.earctic and 

 Nearctic Faun.e : far closer than in any known Tertiaiy or 

 Ouaternaiy stage, were the relations existing in the Holarctic 

 region during the Upper Jurassic period. The resemblances 

 among these minute mammals, as found in the Purbeck of Eng- 

 land and the Como beds of Wyoming are most astonishing, for 

 of thirteen genera, six have their English counterparts, and the 

 family characters are ver}^ close as regards the remainder. (See 

 Osborn, '88, pp. 186-7.) It would be rash to say whether or 

 not continuous close geographical connections existed from the 

 Jurassic throughout the Cretaceous, but in the next Nearctic 

 and Palaearctic parallels which' we can draw, namely in the 

 Basal Eocene between the Torrcjon of New Mexico and the 

 Ccriiaysicn of France, the resemblances are again very close. 

 During the Upper Eocene the faunal parallels became decidedly 

 less close, in fact the correlation can only be established by rel- 

 ativ^ely few forms. In the lower Oligocene the faunal relations 

 suddenly became again much closer between the old and new 

 worlds and they remained close throughout the later Tertiaiy 

 until the middle of the Pleistocene period. 



II. CLASSIFICATION OF THE EUROPEAN TERTIARY 



Invertebrate palaeontologists have naturall}' taken the leader- 

 ship in the classification of the old world Tertiar}^ their vertebrate 

 confreres having followed, adapted and modified the system. 



