4i8 CLASSIFICATION OF THE EQUIDA£. 



in determining the age of horses, especially those which 

 have lost their "mark," is the gradual diminution in the 

 angle which the upper and lower permanent incisors make 

 with each other, as we can see by comparing Fig. 444 

 with Fig. 445. In Fig. 444, which represents the mouth 

 of a four-year-old horse, the angle made by the posterior 

 edges of the two 2nd incisors of the left side, is about 

 180°. The gradual diminution in the acuteness of this 

 angle is due to the continued lengthening of the crowns 

 of the incisors, and to the peculiar shape of the bony 

 sockets (alveoli) in which these teeth are set. As the 

 gain in length, by elongation of the crowns is greater 

 than the loss in length, by wear ; this arrangement of 

 the incisors helps to obviate any dental inconvenience 

 which might arise from excessive growth. We can see in 

 Fig. 643, that the gradual lengthening of the crowns 

 of the molar teeth is a well-marked feature in equine 

 evolution ; and from a study of the mechanism of equine 

 dentition, we learn, that for purposes of food-prehension 

 and mastication, the growth of the incisors must be pro- 

 portionate to that of the molars. Hence we may assume 

 that the crowns of the incisors, like those of the molars, 

 have gradually lengthened during the evolution of the 

 horse ; and consequently, that the acuteness of the angle 

 made by the upper and lower incisors of the horse of to- 

 day, increases with age, to a greater extent than it did in 

 the case of his ancestors. On turning to the domestic 

 ass, we find that in old donkeys (Figs. 448, 449 and 450), 

 the angle in question is greater than in horses of similar 

 ages (Figs. 445, 446 and 447). Consequently we may infer 

 that the teeth of the domestic ass are of an older type 

 than those of the horse. Also, the incisors of the donkey 

 are relatively narrower than those of the horse. 



Differences between Asses and Zebras — The 



tiger-like stripes on the neck and body (hence the name 

 Hippotigris) are perhaps the only distinctive difference be- 



