SOME REMOTE ANCESTORS OF THE HORSE. 
35 
parative anatomist, of the University of Pennsylvania, and it is 
to him we are indebted for our first systematic knowledge of the 
extinct horse family of this continent. He enumerated and 
described no less than nineteen classes of fossil horses, from 
remains collected by the Philadelphia Academy of Sciences. 
Prof. E. D. Cope, of Philadelphia, has also made fossil horses a 
special study for many years, and was the first to discover and 
describe some of the missing links in the horse family. 
In order to give anything like a complete history of the gene- 
ology of the horse, it becomes necessary for us to take a glance at 
the geological formation of the earth. The earth, as you know, 
is divided into epochs, or periods of time; these we enumerate 
from below upwards, and they are as follows: Beginning with 
the first, or primitive formation, we have the Archean, a period 
in which, as yet known, no life had made an appearance on the 
earth. The next epoch is the Paleozoic, or the era of ancient 
forms of life. Next come the Mezozoic, or the era of middle 
life; while fourth and last comes the Cenozoic, or the era of 
more recent life. Now these periods just named are further 
subdivided, but for our purpose it is not necessary for us to 
name only the last. The Cenozoic period is divided into two 
periods, viz., Tertiary and Quarternary. Now these two periods 
are further subdivided into Eocene, Miocene and Pliocen.e. It 
is with these last subdivisions we have here to do, for it is in the 
Eocene formation that mammalia first make their appearance 
on the earth. It is here that we find an animal having distinct 
ungulate characters, namely, the possession of a scapula, and 
feet without claws. This animal antedated the elephant, the 
rhinoceros, tapir, and, in fact, all known hoofed mammalia. 
The name of this ancient beast is Phenacodus, and Cope has 
referred nine species to this genra. Phenacodus had five func¬ 
tional toes on each limb, which was semi-plantigrade. The 
number of teeth was, incisors, 12; canines, 4; premolars, 16, 
and molars, 12, or 44 in all, with a small diastema between the 
canines and premolars. The skull presented the usual charac¬ 
teristics of other Eocene mammalia in the incomplete condition 
