INTERRELATIONSHIPS OF GENERA 



59 



at altitudes above 8000 feet, the other upon the mountains of 

 Guatemala at altitudes above 9000 feet. They have apparently 

 descended from a primitive Phenacomys-like stock and have 



Fig. 31. — Cheek-teeth of Orthriomys urribrosus Merriam. 

 Crown views : a. left upper, 6. left lower molars. 



evolved in the direction of true Microtus; but although they 

 have gone too far in certain respects to be regarded as members 

 of this genus, or as representing its ancestors, the char- 

 acters suggest that all three may have been derived from a 

 common Phe7iacomys-\ike ancestor. The cheek-teeth in both 



Fig. 32. — Cheek-teeth of Herpetomi/s guatemalensis Merriam. 

 Crown views : a. left upper, b. left lower molars. 



Orthriomys and Herpelo»iys have become completely hypsodont 

 and persistently growing; the temporal muscles have become 

 stronger than in the parent form, producing in the adult 

 skull more salient ridges which fuse anteriorly to form a median 

 interorbital crest. In the general character of the enamel 

 pattern of its molars, small size of the auditory bullae, general 



