1879.) Extinct American Rhinoceroses and their Allies, 9714 
Aceratherium. \n Aphelops, the foramen ovale approaches near 
to the f. lacerum, so as tq be separated by a narrow bridge only 
in A. megalodus, which is wanting on one side in a specimen of 
A. malacorhinus. In the genus Rhinocerus, these foramina are 
not divided. 
In the structure of the teeth, the same serial order is to be 
observed. Commencing with the incisors 3 in the tapiroid types 
and Hyracodon, and canine }, we find 3 3 in Zaladis ; 3 $ in Acera- 
therium ; ?= in Aphelops ; + $ in Ceratorhinus and Rhinocerus, 
to 5°, ¢ in Atelodus and Calodonta. As to the molars, in those 
of the upper jaw the series of modifications consists of succes- 
sive complication of the transverse crests. In Alyracodon, as in 
the tapiroid genera, the external wall of the posterior molar is 
Fic. 2.—Aphelops megalodus Cope, inferior view of cranium rep- 
resented in Fig. 1. 
continued beyond the posterior cross-crest; in the Récnocerid@, 
generally the external wall is not continued beyond this crest, 
but is in line with the posterior cross-crest. In a specimen of 
Aceratherium occidentale, the posterior superior molar of one side 
is like that of yracodon, while that of the other side is like that 
of Rhinocerus. The cro$s-crests in Aceratherium are quite simple, 
having slight bulges into the median valley. In the species of 
Aphelops these bulges are more prominent, especially that of the 
posterior crest, which is more externally situated than that of the 
anterior cross-crest, so that the fundus of the valley is turned 
abruptly backwards. In several of the existing species, this 
bulge becomes an antero-posterior crest, and the fundus is fur- 
ther divided by other crests from the outer wall and elsewhere. 
The cingula become so elevated as to cause an isolation of the 
valleys as fosse at a comparatively early stage of wear. This 
