POSITION AND STRUCTURE OF HORSE 33 



the South American extinct Macrauchenia, a member 

 of the same sub-order (Litopterna) as Thoatherium, 

 a genus already referred to in connection with the 

 skeleton of the horse's foot. This shows that the 

 Equidcz are paralleled in two remarkable structural 

 features by Thoatherium in the monodactyle feet, 

 and by Macrauchenia in the presence of pits in the 

 crowns of the incisor teeth. 



Passing on to the cheek-teeth of the horse, it 



A left upper molar of a Single-toed (guus), A, and a Three-toed 

 Horse (Hipparion), B. /, anterior pillar, hy, posterior pillar. 



has to be noted in the first place that these are 

 normally represented in the adult by six pairs in 

 both the upper and lower jaw (pi. v.) ; of which the 

 first three pairs, on account of being preceded by milk- 

 teeth, are known as premolars, while the other three 

 pairs, which have no such temporary predecessors, are 

 the molars, or true molars, as they are often called. 

 In some horses, however, there is a small and 

 practically functionless tooth on the inner side of 



