738 THE EOCENE FAUNA. 



recognized as orders — the Prosimiae and the Insectivora. The latter group 

 has always been a crux to systematists, and when we consider the skeleton 

 alone, as from the standpoint of the palaeontologist, the difficulty is not di- 

 minished. Various extinct types discovered in latter years, chiefly in the 

 Eocene formations, have been additions to this intermediate series of forms, 

 giving even closer relations with the orders already adjacent; i. e., the 

 Edentata, the Rodentia, the Prosimiae, and the Carnivora. As is known, 

 the groups corresponding to these orders have been named respectively the 

 Tseniodonta, Tillodonta, Mesodonta, and Creodonta. With great apparent 

 diversity, these suborders show unmistakable gradations into each other 

 and the two recent orders already mentioned. As such, I may mention 

 Psittacotherium, which relates the Tseniodonta and Tillodonta; Esthonyx, 

 which relates the Tillodonta with nearly all the other suborders; Achce- 

 nodon, which connects Creodonta and Mesodonta, and Cynodontomys, which 

 may be Meso^ont or Prosimian. Then the existing Chiromys most certainly 

 connects Tillodonta and Prosimise. 



My original definitions of the suborders of the Mesodonta, given in 

 Vol II of the United States Geological Survey under Capt. G. M. Wheeler, 

 p. 85, omitted the Prosimiae, and embraced a number of characters whose 

 significance I have re-examined.^ Thus it is impossible to characterize the 

 Creodonta as lacking a trochlear groove of the astragalus, in view of the 

 form of that element in Mesonyx and Mioclanus, where the groove is more 

 or less distinct. It is impossible to distinguish the Insectivora from the 

 Creodonta by the deficiency of canine and large development of incisor 

 teeth. In BJiyncJiocyon the canines are large, and the superior incisors 

 wanting, wdiile in Centefes the arrangement of these teeth is precisely as in 

 the Creodonta. As to the large Achcenodon and other Arctocyonidce, I find 

 no characters whatever to distinguish them from the generally small Meso- 

 donta. 



In view of these inconsistencies, I have re-examined the subject, and 

 have found the following definitions to be more nearly coincident with the 

 natural boundaries of the divisions of this large order. The importance of 



' Sec Proceedings Philadelphia Academy, 1883, p. 77, where the divisions of the Bonotheria are 

 redefined. 



