458 Bulletin America)i Museum of Natural History. [Vol. XXVII, 



CHAPTER XII. CONCLUSIONS. 



I. Genetic Relations of the " Meseutheria" and " C-eneutheria." 



Cope believed that the Basal Eocene (Puerco and Torrejon) fauna was 

 directly ancestral to the Lower Eocene Wasatch fauna and various writers 

 selected different members of the Taligrada, Condylarthra, Creodonta and 

 Tillodontia as ancestral respectively to the Artiodactyla, Perissodactyla, 

 Fissipedia, Pinnipedia, and Rodentia; but Osborn ^ in 1894 (pp. 234-237) 

 advanced the hypothesis that the ""Mesoplacentalia," later called "Mes- 

 eutheria," typified by the Amblypoda, Condylarthra, Creodonta and Tillo- 

 dontia, were not ancestral to the "Ctenoplacentalia" (Ca^neutheria), typified 

 by the Proboscidea, Diplarthra (Artiodactyla and Perissodactyla), Fissipedia 

 and Rodentia, but were the dyino; members of a great Mesozoic Placental 

 radiation; that the Meseutherian orders became extinct on account of 

 "their stationary brain development and comparatively defective tooth and 

 foot structure," while the Cjeneutherian radiation, appearing in the Lower 

 Eocene, began "from some comparatively unspecialized spurs of the dying 

 Mesoplacental group" and was characterized by a "rapidly ])rogressive 

 development of the brain, dentition and feet." 



In so far as this hypothesis implies that the specialized genera of the 

 Meseutheria (e. g., Palwonictis, Patriofelis, Arctocyon, Peripti/chus, Phenaco- 

 dus, Meniscotherium) were not ancestral to the Cseneutherian orders it has 

 been fully confirmed by all subsetpient research by Osborn and others. 

 But the hypothesis has been extended by its author so that, if the writer 

 correctly understands Professor Osborn, it now means: (1) that the Cseneu- 

 therian orders have been derived from some wholly unknown ])otentially 

 large brained stock; (2) that no Creodonts were ancestral to the Fissipedia, 

 no Condylarths to the Perissodactyla, no Amblypods or Condylarths to the 

 Hyracoidea, Embrithopoda, Proboscidea or Notoungulata.- 



The conclusions whic-h seem to be indicated by the evidence now avail- 

 able are as follows: 



(1) The large brained "Cieneutheria" have been derived from forms 

 in which the brain was of the Meseutherian type. Unless the evidence of 



1 Trans. N. Y. Acad. Sci., Vol. XIII, 1894. 



2 Professor Osborn has kindly stated his present view in the following words: 



"My point has always been: 1) that the known Meseutheria are not an ancestral hut a dying 

 out group, 2) that small brained forms elsewhere — with a potential of brain development — gave 

 rise to the rapidly progressive Ceneutheria. Such forms may come within our definition of 

 Creodonta but more likely within that of Insectivora." 



