4 88 



UNGULATES. 



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. ^ horses 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 groups have 



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 p 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 

 in the males of all the living species is 42 ; these comprising § incisors, 

 \ canines, 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 FALATAL VIEWS OF SKULL OF EXTINCT THREE-TOED 



horse. — After Zittel. 



Equus. 



