374 M. R. Thorpe — Arceocyon. 



example, the mastodons of both Europe and North Amer- 

 ica in the Pliocene were of two kinds, that is, some had 

 three ridges on the intermediate molars (trilophodont) 

 while the others were tetralophodont. 



The order Carnivora are unquestionably of Holarctic 

 origin, according to Matthew, and from this area they 

 have spread to practically all parts of the world. The 

 ancestors of Simocyon are obscure, but it would be reason- 

 able to suppose that they may have lived in Africa, owing 

 to the close similarity between the faunas of that conti- 

 nent and of Pikermi and Eppelsheim. On account of the 

 considerable contrast between the Eppelsheim S. dia- 

 phorus and the Pikermi S. primigenius, it would appear 

 that the northern species was aberrant and became extinct 

 before or soon after the beginning of , the Pliocene, where- 

 as the southern form represented the stem stock and 

 followed the faunal migration through India and China 

 and across the Bering Strait land connection into North 

 America. It is probable that there might have been a 

 concentration of carnivore types in the region of this 

 "game trail." It may have been some such concentra- 

 tion of mammals in a restricted area which produced the 

 rise of the Carnivora in the beginning. 



The early Pliocene is known to have been mild of cli- 

 mate, but during the entire epoch there was a very 

 gradual cooling or lowering of temperature which slowly 

 drove the animals southward, and may account for the 

 presence in Oregon in Middle Pliocene times of this 

 simocyonid. 



It would seem to be a reasonable explanation to suppose 

 that the above line of migration was the one followed by 

 this phylum of carnivores from southern Europe to North 

 America. It also could account for the differences and 

 similarities in structure between the Old and the New 

 World species of this same phylum, if we take into con- 

 sideration the great lapse of time between Upper Miocene 

 and Middle Pliocene. 



The systematic position of Arceocyon and Simocyon 

 ought to be considered together, for I think that they both 

 were probably derived from the same genus. There are 

 several structural characters which prevent us from con- 

 sidering these forms as being at all closely related to 



