18 



MICROTIN^ 



Dentition. 



As usual in Muridse the dentition of the Microtinse consists 

 of sixteen teeth, namely a pair of incisors and three pairs of cheek- 

 teeth or molars in each jaw above and below, usually expressed 

 by the formula i\ m| = 16. It is, however, questionable 

 whether the three cheek-teeth are in fact the homologues of the 

 teeth called m\, to|, and mf in other placentals (see p. 124). 



As in all other Rodents the incisors are persistently growing 



Fig. 9. — Vertical longitudinal sections of skulls of Muridae (enlarged). 

 a. Raltus raltus Linnaeus; 6. Neotoma sp. (See p. 16.) 



teeth. The upper incisors are strongly curved as compared with 

 those of the lower jaw and form larger segments of relatively small 

 circles, whereas their opponents invariably form smaller segments 

 of relatively large circles. Posteriorly the alveolar sheath of 

 each upper incisor passes backwards into the maxillary bone, 

 where it usually terminates just in front of the alveolus of m^; 

 in Ellohius, however, it pushes its way back on the inner or lingual 

 side of the molar roots to terminate in the hinder part of the 

 maxilla immediately dorsal to the palatal surface of the bone, the 

 termination of the tooth capsule being often marked by a fenestra- 

 tion of the maxilla. Each lower incisor passes backwards through 

 the mandibular ramus. In the Lemmings this tooth lies on the 



