212 PROF. J. LOGAN LOBLEY, F.G.S., F.R.G.S., ON 
America six species of Equus have been described, and in the 
South American bone caves the genus is also well represented. 
But this is by no means all, for when mammalian fossils of the 
older Tertiary deposits are examined it is found that horse-like 
animals lived quite through Tertiary times to as far back as 
Lower Eocene. And it is a very important scientific fact that 
the differentiation of these fossils from the present equine type 
of Europe and Asia, Equus caballus, increases with the 
geological age of the deposits in which they occur. So 
markedly is this the case that, from American fossils, a 
chronological series of genera of Equidse gradually approxi- 
mating to Equus cctbcdlus can be constructed. The older the 
genus the smaller the animal and the more unlike is the foot to 
the single-toed hoof of the horse. 
Equus. 
Series of American Equidce. 
Single-toed hoof. 
Quaternary. 
Pliohippus. 
Size of ass, single- 
Pliocene. 
Protohippus. 
toed. 
Two small lateral toes 
)) 
Hipparion. 
but functionless. 
)) 
)) 
Auchippus. 
)) 
Meryehippus. 
>> 
i) 
Myohippus. 
>) 
Anchitherium. 
Size of large goat, 
Miocene. 
Miohippus. 
three toes. 
5 J 
>> 
Mesohippus. 
JJ 
)) 
Orohippus. 
Size of fox, four toes. 
Eocene. 
Eohippus. 
Eour toes and one 
Phenacodus. 
undeveloped. 
Eive toes. 
Lowest Eocene. 
There are also forms such as the Parahippus, the Lophiodon, 
and the Palaeosyops, which are more generalised and partake of 
the characters of both Hippidae and Tapiridae. 
Again, the rhinoceros is not represented in the present' 
American fauna, but in Pliocene and Miocene times the family 
Bhinoceratidae was represented in America by the genera 
Bhinoceros, Aceratherium, Hyracodon, and Dicer athcrium. The 
remarkable extinct family Brontotheridae, some of the members 
of which attained the size of elephants, with four toes to the 
front and three to the hind feet, has been given to science by 
the Miocene of Colorado. 
