771\c Extinct American Rhinoceroses and their Allies. [December, 
The posttympanic process is, it is well known, well separated 
from the postglenoid process in the .tapir, so as to leave the 
auditory meatus widely open below.. The arrangement is similar 
in Hyracodon. In Rhinocerus,as shown by Flower, the meatus is 
closed below by the codssification of the two processes. In the 
oldest genus of the family Aceratherium, the relations of the parts 
are asin Hyracodon. In Aphelops the two processes approach 
each other, but do not come in close contact as in the genus 
Ceratorhinus. 
A, 
Fic. 1.—Aphelops megalodus Cope, one-sixth natural size. Loup Fork 
beds, Colorado. 
The postglenoid process is low and transverse in the tapirs; in 
Rhinocerus it is long and has a triangular section. In some spe- 
cies of American Aceratheria its form is much like that of the 
tapirs (4. mite, A. occidentale); while in Diceratherium pacificum 
and in the species of Aphelops, the form of this process is as in 
Rhinocerus. 
In the tapirs, the foramina sphenoorbitale and rotundum are dis- 
tinct. They are also distinct in Aceratheriuin mite. In D. pacifi- 
cum they are confluent, but the walls of their orifice present two 
opposite projections, which are the rudiments of a dividing sep- 
tum. In Aphelops these foramina are one as in Rhinocerus. At 
the same time, the external wall of the alisphenoid canal is 
shorter and thinner in the Aceratheria than in the Aphe/opes. 
In the older types of Perissodactyla, e. g., Symborodon, the 
foramen ovale is situated well in advance of the foramen lacerum 
medius, and is separated from it by a considerable space of the 
sphenoid bone. The same structure is seen in Hyrarcadan and in 
