FOSSIL VERTEBRATES— MAMMALIA 1 79 



in their dental succession, which places them apart as a distinct 

 side phylum. This does not affect the derivation of the Creo- 

 donta themselves from stem forms of unspecialized Insectivora 

 existing in the Jurassic period, the characters of which are seen 

 in the Insectivora Primitiva, or placentals of the Stonesfield Slate 

 and Purbeck periods." 



The Creodonta are generally divided into eight families, which 

 are here arranged as nearly as possible in the order of their 

 evolution, which was directed toward the development of more 

 perfect sectorial teeth and more and deeper convolutions on the 

 surface of the brain. 



Arctocyonidae : from the lowest Eocene of the United States, 

 Puerco and Wasatch and the lowest Eocene of France. 



Oxyclcznidae : from the lowest Eocene of the United States. 



Triisodontidae : from the lowest Eocene of the United States. 



Mesonchyidae : from the Lower and Middle Eocene and possi- 

 bly from the Miocene of the United States. 



Proviverridae : from the Eocene of Europe and the United 

 States. 



Paleonictidae : from the Lower Eocene of the United States. 

 This family is of some interest as containing the possible ances- 

 tor of the pinniped group of the true Carnivores. 



Hyce-nodontidae : from the Eocene to the Miocene of Europe 

 and America. This family has well-developed teeth of the sec- 

 torial type and approaches very close to the true carnivores. 



Miacidae: from the Eocene of the United States. This family 

 so closely approaches the modern carnivores that they have 

 been placed among them by certain authors. 



From the Lower Tertiary deposits of South America, Pata- 

 gonia, come many forms that are undoubtedly Creodonts but of 

 doubtful position in the suborder ; by some authors they are 

 placed among the Hycenodontidae, and by others they are placed 

 in a separate group, the Sparrassodonta. The best known 

 forms are Prothylacinus and Borhycena ; they are very similar 

 in some respects to the carnivorous Marsupial, Thylacinus, of 

 Tasmania. 



