UNGULATES. 
488 
while in the hind-limb the remnant of the fibula becomes similarly fused with 
the tibia. 
So far as their extremely specialised organisation is concerned 
Specialisation. ^ p orses hold a position among the Odd-toed Ungulates precisely 
analogous to that occupied by the true Ruminants, or Pecora, in the Even-toed 
division of the order; and it is curious to observe how the two gioups ha\e 
undergone an almost 
exactly parallel de¬ 
velopment, although 
differing so essentially 
from one another in 
the structure of their 
limbs and teeth. In 
both, for instance, the 
cheek-teeth have ac¬ 
quired tall crowns, 
with complicated fold¬ 
ings of the enamel, and 
the front teeth are 
separated by a long 
gap from those of the 
cheek - series. Then, 
again, both have the 
feet supported by a 
long cannon-bone, 
although in the Ruminants this is formed by the fusion of two distinct elements, 
and in the horses of but one original constituent. Moreover, both groups have 
the two bones of the lower segment in the fore and hind-limbs fused together, 
and in both the process by which the second vertebra of the neck articulates 
with the first has assumed a spout-like form. 
The whole of the existing representatives of the horses are 
included in the genus Equus, of which the following are dis¬ 
tinctive characteristics. In the upper cheek-teeth the portion called the anterior 
pillar (marked jp in the middle figure on p. 487) is connected by a narrow neck of 
enamel and ivory with the adjacent crescent in the middle of the same side of 
the tooth; and each foot has but a single toe. The total number of teeth 
the males of all the living species is 42; these comprising § incisors, 
anines, and ^ cheek-teeth. The first upper tooth of the cheek-series, that 
is to say the first premolar, is, however, very small in all the living species, 
and is frequently wanting, thus reducing the number of teeth to 40. It is, 
however, larger in many fossil species, and a few of these also have a small 
first premolar in the lower jaw, thus bringing up the number to the typical 
44. The canines of the males are rather small and pointed, and in the 
females are either rudimentary or wanting. When present, they are placed 
near to the incisors, but are separated by a long gap from the teeth of the 
cheek-series. 
SIDE AND PALATAL VIEWS OF SKULL OF EXTINCT THREE-TOED 
horse.—A fter Zittel. 
Equus. 
m 
1 
1 
