430 THE VERTEBRATE SKELETON. 



the dentition is brachydont and bilophodont, the grinding 

 teeth in general resembing the posterior upper molars of the 

 Tapir. The same type of brachydont tooth is seen in Palaeo- 

 therium but the transverse ridges are crescentic instead of 

 straight, and are separated from one another by shallow 

 valleys without cement. Some of the Palaeotheridae have the 

 regular series of forty-four teeth. 



A complete series of forms is known showing how from the 

 simple brachydont teeth of the Palaeotheridae, were derived 

 the complicated hypsodont teeth of the Equidae. The increase 

 in depth of the tooth was accompanied by increase in the 

 depth and complexity of the enamel infoldings, and of the 

 cement filling them. 



Both upper and lower grinding teeth of the Equidae are 

 much complicated by enamel infoldings, but their derivation 

 from the bilophodont type can still be recognised. The 

 diastema in front of the premolars is longer in the living 

 Equidae than in their extinct allies. In the adult horse the 



31 33 



dental formula is i - c -pm-m-, total 40, with often a 

 o 1 66 



vestigial first upper premolar (fig. 82, pm 1). The last molar is 

 not more complex than the others, and in the female the canine 

 is quite vestigial. The incisors are large and adapted for cutting 

 and have the enamel curiously folded in forming a deep pit. 



30 3 



The milk dentition is di -= dc - dpm - , total 24. The last 



o 6 



milk premolar is not more complex than the premolar that 

 succeeds it. The horse affords an excellent instance of a 

 typically herbivorous type of dentition, the cutting incisors, 

 reduced canines and series of large square flat-crowned grinding 

 teeth being most characteristic. 



In Rhinoceros the grinding teeth are much like those of 

 Lophiodon, having an outer longitudinal ridge from which two 

 crescentic transverse ridges diverge. The upper premolars are 

 as complex as the molars, and there are no canines ; in some 



