THE TEETH OF PERISSODACTYLE UNGVLATA. 323 



growth from a persistent pulp, and, in fact, happens after 

 the formation of its roots. 



The pattern of the horse's molar has been already de- 

 scribed ; it should be added that the last molar differs from 

 the rest in its posterior moiety being less developed than in 

 the other teeth. 



As each ridge and each pillar of the tooth consists of den- 

 tine bordered by enamel, and the arrangement of the ridges 

 and pillars is complex ; as, moreover, cement um fills up the 

 interspaces, it will be obvious that an efficient rough grinding 

 surface will be preserved by the unequal wear of the several 

 tissues. 



When a bit is put into a horse's mouth it rests in the 

 interval, or diastema, which exist between the incisors and 

 the commencement of the molar series, and the great con- 

 venience of the existence of such a space has led many 

 authors to assume that the horse was moulded in accordance 

 with man's special requirements, so that it might be suited 

 for its subserviency to his wants. 



But the wide diastema appeared in the remote ancestors 

 of the horse long ages before man's appearance on the earth, 

 and the advocates of this theory of design would, as Professor 

 Huxley suggests, have to tell us what manner of animal rode 

 the Hipparion. 



The milk teeth of all the Ungulata are very complete, 

 and are retained late ; they resemble the permanent teeth 

 in general character, but the canines of the horse, as might 

 have been expected, their greater development in the male 

 being a sexual character, are rudimentary in the milk 

 dentition. 



To the Perissodactyle Ungulates which are specially inte- 

 resting on account of their dentition, must be added Homa- 

 lodontotherium, a tertiary mammal, the remains of which were 

 described by Professor Flower (Phil. Trans., 1874). 



It had highly generalised characters ; its teeth were 



Y 2 



