INTERRELATIONSHIPS OF GENERA 



63 



auditory bullae contain spongy tissue. Externally Proedromys is 

 a very ordinary looking vole with long coarse fur, a moderately 



Fig. 36. — Microtus ratticeps K. & Blasius. 



Dorsal, ventral, and lateral views of skull, enlarged ; the small figure 

 shows the skull in dorsal view, natural size. 



long tail, and six plantar tubercles. The mammary formvda 

 is 2 — 2 = 8. 



Microtus is the largest and the most widely distributed 



