Cope.J 6v^ [April 15, 



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 bulle- 

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



This series may be represented in genealogical relation, as follows :* 



C<xlodonta. 



/ 

 Bhinocerus. Atelodus. 



\ / 



Ceratorhinm. Peraceras. 



\ / 



Aphelops. 



Zalabis. Ccenopus. Dieeratherium. 



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

 which preceded both it and the Aceratheria in time, is the genus Triplopus 

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

 the incisors are probably f and the canines |. This formula is that 

 of the Eocene tapirs, where the normal numbers | \ prevail. Ti'iplopus 

 further dilfers in the primitive condition of the premolars above, which, as 

 in the LopModontidm, differ from the molars in their greater simplicity. 

 Thus it is probable that tapiroids, probably LopModontidm, gave origin to 

 the Bhinocendm, as Marsh has suggested. And it is further altogether 

 probable that the general type of dentition presented by the RMnoceridm, 

 LopModontidoB, etc., which I have named the palseotheriodont, took its 

 origin from the type which is intermediate between it and the bunodont, 

 viz, the symborodont, as I have pointed out in an essay on this subject. 



The first appearance of dermal horns was apparently in a pair placed 

 transversely on the nasal bones, in species of Eocene LopModontidm of the 

 genus Colonoceras. The same character has been observed by Marsh in 

 species of the Lower Miocene, which probably belong to the true Bliino- 

 ceridm, and which he has called Dieeratherium. This genus appears to 

 have terminated the line exhibiting this structure, and the family in North 

 America remained without horn. As we have seen, the types possessing 

 the median horn arose in Europe, in the GeratorMmts scMeiermaclieri of 

 the Middle Miocene, and still survives. 



*See American Naturalist, 1880, p, 611. 



