692 ' THE EOCENE FAUNA. 



Incisors \; canines f; a tuberosity for a dermal horn on each nasal bone. 



Diceratherium. 



Incisors \; canines f ; a median dermal nasal horn Ceratorhinus, 



Incisors f ; canines f • - - Zalabis. 



Incisors^; caniues ^; dermal horn median; no osseous nasal septum Atelodus, 



aa. Posttympanic process coosified with postglenoid; 



Incisors \; canine ^; dermal horu median; uasal septum not ossified Bhinocerus. 



Incisors g ; canine ^ ; dermal horu median; nasal septum ossified Ccelodonta. 



It can readily be seen that the genera above defined form a graduated 

 series, the steps of which are measured principally by successive modifica- 

 tions of four different parts of the skeleton. These are, first, the reduction 

 of the number of the toes of the anterior foot; second, the reduction in the 

 number and development of the canine and incisor teeth; third, the degree 

 of closure of the meatus auditorius externus below; and, fourth, in the de- 

 velopment of the dermal horns of the nose and its supports. While these 

 characters have that tangible and measurable quantity which renders them 

 available for generic diagnosis, there are others which possess a similar 

 significance, and which I have noticed in an article published in the Bulletin 

 of the U. S. Geological Survey of the Territories for September, 1879. 



This series may be represented iu genealogical relation as follows:^ 



Coelodonta, 



/ 



Bhinocerus. Atelodus. 



\ 



Ceratorhinus. Peracerds. 



Aphelops. 



Zalabis. Ccenopus. Diceratherium. 



The early type, which corresponds most nearly with Ccenopus, and 

 which pi-eceded both it and the Aceratlieria in time, is the genus Triplopus 

 Cope, which has left a species in the Upper Bridger of Wyoming. Here 

 the incisors are probably 3 and the canines \. This formula is that of the 

 Eocene tapirs, where the normal numbers | \ prevail. Triplopus further 



' See American Naturalist, 1880, p. 611. 



