52 



THE HORSE 



if the matrix, or portion of the skin to which it is 

 attached, is much injured, it may assume a more or less 

 irregular shape. 



As regards the dentition, the incisors are variable, 

 generally reduced in number, and often quite rudimen- 

 tary and entirely disappearing at a very early age. 1 The 

 canines in existing species are absent. 2 In respect to 

 the front teeth, therefore, a very marked amount of 

 specialisation has taken place. On the other hand, the 

 cheek teeth are retained in full normal numbers — viz. 

 four premolars and three molars on each side above and 

 below, all in contact, and closely resembling each other, 

 except the first, which is much smaller than the rest, 

 and often lost early in life. The others gradually 

 increase in size from before backwards up to the penul- 

 timate, which is the largest. The upper molars have 

 a very characteristic pattern, admirably adapted for 

 bruising and crushing coarse vegetable substances, and 

 which is clearly a modification of the pattern already 

 seen in the corresponding teeth of Hyracotherium. The 

 lower molars are of simpler form, the two transverse 

 ridges being curved into a crescentic form. In neither 



1 It is difficult to see what advantage the great African two- 

 horned rhinoceroses can find in the complete absence of their front 

 teeth, but this is one of those numerous cases in which we must be 

 content to acknowledge our ignorance and wait for the explana- 

 tion. 



2 It should be stated that certain teeth, regarded above as 

 incisors, are considered by some zoologists as modified canines. 



