INTERRELATIONSHIPS OF GENERA 



41 



highly specialized forms, colonizes the condylar process. If we 

 admit that the great backward extension of the incisor is a more 



Fig. 21. — Ondatra zihelhica Limiaeus. 



Skull and right mandibular ramus dissected to show the alveolar courses 

 of the teeth; the bottom figure shows the left mandibular ramus 

 dissected from the outer side (enlarged). 



ancient feature in voles than is the hypsodonty of the molars, 

 the peculiar relation of the molars to the lower incisor, just 

 described and so characteristic of all Microti, is easily explained ; 



