66 



MICEOTIN^ 



fall for detailed treatment under the genus. None of the species 

 is specially modified for fossorial or aquatic habits. All have 

 normal coats, moderately developed eyes and ears, and normal 

 hands and feet, with claws of normal length, those of the hind-foot 

 being the longer, with moderately hairy soles and with six plantar 



Fig. 39. — Cheek-teeth of Microtus roherii Thomas. 

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



tubercles ; the tail is short, or of medium length, and the mammary 

 formula is 2 — 2 = 8 (except in the small mexicanus group, where 

 it is reduced to 1 — 1 = 4). The skull is normal, varying in 

 shape, massiveness, and angularity with the species. The 

 temporal ridges, stronger or weaker, fuse (at all events in old 

 age) in the interorbital region; the post-orbital processes of the 



Fig. 40. — Cheek-teeth of Microtus calamorum Thomas. 

 Crown views : a. right upper, h. left lower molars. 



squamosals are more or less developed and an anterior encroach- 

 ment of the squamosals upon the frontals is more or less evident. 

 The palate is of normal type in all, and in some forms becomes 

 highly specialized by the extension of the post-palatal pits and 

 the attenuation of the postero-median septum. The pterygoid 

 fossae are deep. The auditory bullae contain spongy tissue and 

 the stapedial artery is enclosed in a bony tube. The mandible 



