834 SLRUDIES: HOR SRODEN LS 
Amynodontide: Aquatic Rhinoceroses ; Amynodon and Cadurco- 
therium, manus functionally tetradactyl; incisors atrophied ; 
upper and lower canines greatly enlarged. 
Rhinocerottdae: True Rhinoceroses; Aceratherium and Rhi- 
noceros ; manus functionally tridactyl; upper canines atrophied ; 
median upper incisors and lower canines opposed and irregularly 
developed. 
Our knowledge of the three divisions of this superfamily 
extends back only to the Middle Eocene of America and 
Europe, namely, to the Bridger and the somewhat older Eger- 
kingen Beds of Switzerland. No Rhinoceroses of any kind have 
as yet been found contemporary with the primitive Horses and 
Tapirs of the Wasatch of America or the Suessonian of France, 
but they will undoubtedly be discovered in these or older rocks 
either in America or Europe, with characteristics as sharply 
defined as those of the other perissodactyl families. 
Certainly before the Middle Eocene of North America, the 
Rhinocerotoidea had here or in some unknown region special- 
ized and diverged into the three above-mentioned families, which 
some authors place in the single family Rhinocerotide. While 
it is quite possible that in the Wasatch or Suessonian period this 
group consisted of a single family, in the Bridger we certainly 
find two distinct families, the Hyracodontide and Amynodon- 
tide, and in the White River these coexist with the Acerathe- 
riinz, or ancestral true Rhinoceroses. The members of each 
family were evidently as widely different in their external form 
ASMMEEMenadentalanGdskeletalmstnuc tues aie 
There is no doubt, therefore, that as a matter of taxonomic 
clearness as well as of phylogenetic fact it is best to consider 
these three families as entirely separate and undergoing a paral- 
lel development, probably in Europe as well as in America. 
Specialization in habits —The wide separation of these three 
families will be fully apparent after we have examined their chief 
primitive, parallel, and divergent features. Parallelism is mainly 
confined to the evolution of the molar teeth, for in every feature 
of the incisor teeth, the skull, the vertebre, and the limbs, these 
