18*1.] 9 [Cope. 



Measurements. M. 



Diameter of canine (transverse) 027 



of P-m. ii 007 



Depth of ramus at P-m. iii 070 



at M. i 085 



" '* " at front of M. iii. . 090 



The matrix in which this jaw was found, is much like the Upper Plio- 

 cene material of Tequixquiac. It is therefore of probably later age than 

 the true Aphelops fossiger, which is a characteristic Loup Fork species. 

 Leidy describes (Extinct Fauna of Dakota and Nebraska, p. 230) a rhi- 

 noceros, probably an Aphelops, from California, under the name of R. 

 hesperius. It is smaller than the Toluca specimen, but has a considerably 

 longer diastema. Its geological horizon is uncertain. 



I mention here that rhinoceroses, probably of the genus Aphelops, ap- 

 parently existed in North America during the Pliocene period. Bones of 

 a species having resemblances to the A. fossiger have been sent me by my 

 assistant, George C. Duncan, from the Equus beds of the eastern part of 

 the Oregon desert. The genus has been hitherto supposed not to ascend 

 higher than the Loup Fork, or Upper Miocene beds. These bones are 

 accompanied by teeth of a peculiar Hippotherium unlike those of any spe- 

 cies of the genus known to me from the Loup Fork Miocene. 



EQUUS Linn. 



The remains of horses are very abundant in the valley of Mexico,* and 

 represent four species. In the determination of these species it has be- 

 come necessary to compare them with those hitherto found in North and 

 South America. In making this comparison I exclude the species of 

 Hippidium, which are all American, and whose molar teeth are easily dis- 

 tinguished by the equality in size of the internal columns ; resembling in 

 this respect the genus Protohippus. 



When the species of the genus Equus differ in the characters of their 

 superior molar teeth, the diversity is to be seen in the size and form of the 

 anterior internal column. The anteroposterior diameter of this column, 

 as well as the integrity or emargination of the internal border of its section, 

 varies according to the species. The infolding or the borders of the lakes 

 has a value, but a less constant one. The Equus eaballus differs from all 

 of the American extinct species, where the corresponding parts are pre- 

 served, in the great elongation of the face, which is expressed in the 

 greater lengths of the diastemata anterior and posterior to the canine tooth 

 in both jaws. Other characters may be observed in the relative lengths 

 of the limb bones, the form of the occiput, etc. It has been shown by 

 Leidy, Riitimeyer and others, that it is not always practicable to distin- 

 guish the species of horses by their teeth alone. A glance at Owen's 



* This fact has already been made known by Von Meyer, Palseontographica, 

 1867, p. 70, and Owen, Transactions of the Royal Society, London, 1869. 



