W7\g Lxtinct American Rhinoceroses and their Allies. [ December, 
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 Duvernoy in species of the Lower Mio- 
cene, which belong to the true Rhinoceride, and which Marsh has 
called Diceratherium, This genus appears to have terminated 
the line exhibiting this structure, and the family in North Amer- 
ica remained without horn. As we have seen, the types possess- 
ing the median horn arose in Europe, in the Ceratorhinus schleter- 
machert of the Middle Miocene, and still survives. 
It may be observed in conclusion, that a successive increase of 
size in the species of this line has taken place in North America 
with the advance of geologic time. Thus, their probable ances- 
tors of the genus Hyrachyus were the least of all. The Acera- 
Fic. 4.—Aphelops megalodus Cope, skull from above (same as Fic. 1), 
one-sixth.natural size, 
therta of the White River formation were larger, the oldest, A. 
mite, being the smallest. The Diceratherta of Oregen were larger 
still. The species of the Loup River or Upper Miocene forma- 
tion were larger, and nearly equal to the large existing species. 
Aceratherium Kaup. is characteristic of the Miocene or Middle 
Tertiary formations of Europe, and is the primitive form of the 
true rhinoceroses. Its four anterior digits relate it to the lower 
or more generalized perissodactylous types of the same and of 
older geological horizons, which are equally allied to the tapirs. 
The dentition differs from that of the genus Rhzzocerus in the 
presence of two superior incisors, but agrees with it in the exis, _ 
ence of one incisor and one canine on each side below, and in t. 
forms of the premolar teeth. The species display great simplicic, 
in the character of the crzsts of the molars. They also possess, . 
