196 MAMMALIAN DENTITION 



both upper and lower jaws and the third molar in each case is 

 the smallest. Both premolars and molars are hypsodont and 

 resemble the corresponding teeth of Dolichotis but the columns 

 are much more closely pressed together so that the anteroposte- 

 rior diameter of each tooth is much less than the transverse. The 

 teeth all have permanent pulps, exhibit enamel on the anterior 

 face only and show the curve characteristic of an advanced 

 Rodent dentition. In one respect the Rabbit has progressed far 

 beyond any of the Simplicidentata for in all Rodent types so 

 far considered the occlusion of upper and lower cheek teeth 

 of one side necessarily involved occlusion upon the other also: 

 it is not so in the Rabbit in which the distance between the two 

 rows of mandibular cheek teeth is less than that between the 

 rows of teeth in the upper jaw. This condition is also observed 

 in the Ruminants. In the Rabbit, mastication can proceed upon 

 one side only of the mouth at a time; upper and lower cheek 

 teeth are not in occlusion upon both sides at once. 



Upon tracing back the cheek teeth of the Hares in paleon- 

 tological history it has been found that the enamel was not al- 

 ways confined to the anterior aspect of the tooth, that the deep 

 notches on the internal and external faces are also secondary, 

 that the single root with its permanent pulp was preceded in 

 geological times by a three-rooted type and that the whole tooth 

 was formerly less hypsodont. Already in the Oligocene the ex- 

 tent of the enamel was becoming reduced and the two lateral 

 roots growing more rudimentary whereas the internal attained 

 a proportionately greater size and importance. The trituber- 

 cular form of the upper molar has become obscured in the 

 course of time and the complexity of the crown increased by 

 the addition of a large cingulum which composes the inner part 

 of the tooth. The cheek teeth of the milk dentition still exhibit 

 vestiges of a tritubercular crown and this makes it the more 

 probable that the permanent teeth were of a similar appearance. 

 In spite of the present peculiar pattern therefore we may feel 

 confident that before the ancient separation of Rodents into 



