Cope.| 394 [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 auterior 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. 8. Geological Survey of the Territories for September 1879. 
This series may be represented in genealogical relation, as follows :* 
Celodonta. 
/ 
Rhinocerus. Atelodus. 
Ceratorhinus. Peraceras. 
% / 
Aphelops. 
Zalabis. Cenopus. Diceratherium. 
The early type, which corresponds most nearly with Canopus, and 
which preceded both it and the Aceratheria in time, is the genus 7riplopus 
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 Kocene tapirs, where the normal numbers #4 prevail. TZriplopus 
further differs in the primitive condition of the premolars above, which, as 
in the Lophiodontida, differ from the molars in their greater simplicity. 
Thus it is probable that tapiroids, probably Lophiodontide, gave origin to 
the Rhinocerrde, as Marsh has suggested. And it is further altogether 
probable that the general type of deutition presented by the Rhinoceride, 
Lophiodontide, etc., which IJ have named the paleotheriodont, 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 Lophiodontide of the 
genus Colonoceras. The same character has been observed by Marsh in 
species of the Lower Miocene, which probably belong to the true Rhino- 
ceride, and which he has called Diceratheritum. 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 Ceratorhinus schleiermachert of 
the Middle Miocene, and still survives. 
*See American Naturalist, 1880, p, 6L1. 
