346 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



the loss of the canine and of all but one, or rarely of 

 two, of the premolars, which leave a wide diastema ; 

 and the posterior position of the molar teeth, as relates 

 to the rest of the skull. A peculiarity which belongs to 

 the highest types of the order is the prismatic form of 

 the molars, and the deep inflection of their always 

 transverse enamel folds both laterally and vertically. 

 A peculiarity of the masticating apparatus, which is the 

 basis of distinction from the bunotherian order, is the 

 lack of postglenoid process, and the consequent freedom 

 of the lower jaw to slide backwards and forwards in 

 mastication. Appropriately to this motion the condyle 

 of the mandible is either subglobular, or is extended 

 anteroposteriorly, and the glenoid cavity is a longitudi- 

 nal instead of a transverse groove. 



The mechanical action of the development of the 

 rodent dentition has been as follows. The first factor 

 in the order of time and importance was the increasing 

 length of the incisor teeth. Those of the lower jaw 

 closed behind those of the upper in the progenitors of 

 the Glires (e. g. Psittacotherium) as in other Mamma- 

 lia. Increase of length of these teeth in both jaws would 

 tend to keep the mouth permanently open, were it not 

 for the possibility of slipping the lower jaw backwards 

 as it closed on the upper. This backward pressure 

 had undoubtedly existed, and has operated from the 

 earliest beginning of the growth of the rodent incisors. 

 The process has been precisely the opposite of that 

 which has occurred to the Carnivora, where the pres- 

 sure has been ever forwards owing to the development 

 of the canines. The progressive lengthening of the 

 incisors through use has been dwelt on by Professor 

 Ryder (/. <:.). The posterior pressure on the lower 

 jaw, produced by its closing on the upper, has been 



