INTERRELATIONSHIPS OF GENERA 



55 



only five plantar tubercles. Flank glands are present in adult 

 males. In the typical subgenus the mammary formula is 

 reduced to — 2 = 4 ; in the subgenus Micrurus, which is 

 slightly less modified, the mammary formula is 1 — 2 == 6. 



Fig. 27. — Cheek-teeth of Pitymys majori Thomas. 

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



The general external characters have thus been considerably 

 modified in relation to the subterranean habits of these voles. 



The skull is more or less evidently modified for fossorial life, 

 usually with a rather smooth, delicately built, more or less 

 depressed braincase. The temporal ridges are weakly developed 



Fig. 28. — Cheek-teeth of Pitymys ibericus Gerbe. 

 Crowa views : a. right upper, b. left lower molars. 



and are widely separated in the iuterorbital region ; the post- 

 orbital crests of the squamosals are usually feeble. The palate 

 is normal and the pterygoid fossae are deep. The auditory bullae 

 are usually swollen, their mastoid portions also being inflated; 

 the walls of the buUos are strengthened by spongy tissue and 

 the stapedial artery is enclosed in a bony tube which passes 

 through the stapes. The mandible and incisors are normal. 

 The cheek-teeth are rootless, with normally differentiated 



