1879.) Extinct American Rhinoceroses and their Allies. 771d 
Aceratherium. In Aphelops, the foramen ovale approaches near 
to the f. lacerum, so as to 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 2 $ in Zaladis ; 2 ¢ in Acera- 
therium ; 2 { 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 /yracodon, as in 
the tapiroid genera, the external wall of the posterior molar is 
Fic, 2.—Aphelops MR Cope, bane view of cranium rep- 
nted in 
continued beyond the posterior cross-crest; in the Rhinoceridæe, 
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 Hyracodon, while that of the other side is like that 
of Rhinocerus. The cross-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 — 
male as fossæ at-a gamete early ge of wear, This 
