50 



MICROTIN^ 



from some close ally of Mimomys intermedius. The cheek-teeth 

 have become persistently growing, and m^ is a little reduced ; 

 but otherwise their pattern and particularly that of the mj, is 

 exactly as in adult M. intermedius. In the skull the temporal 

 ridges fuse to form a median interorbital crest, but the squa- 

 mosals, which have small but rather conspicuous post-orbital 

 crests, remain widely separated anteriorly. The palate is essen- 

 tially as in normal Microtus, but its postero-median sloping 

 septum is rather short and broad. Traces of the median suture 

 of the palate and of the sagittal suture of the interparietal some- 

 times persist. The auditory bullee are well inflated and their 

 walls are strengthened by a thick development of dense spongy 

 tissue ; the mastoid portion is also slightly inflated, and the 

 stapedial artery passes through the stapes enclosed in a bony 

 tube. Externally Phaiomys is evidently modified for fossorial 



Fig. 23. — Phaiomys leucurus Blyth. 

 Crown views of cheek-teeth : a. right upper, 6. right lower molars. 



habits and for life in its special station; the fur is peculiarly 

 long and soft; the tail is shortened and densely clothed; the 

 palms and soles are overgrown with hair, and the claws, both 

 of the hands and feet, have been considerably lengthened. 



The North American genus Phenacomys appears to repre- 

 sent, in its essential characters, the common stock from which 

 two of the most important groups, namely, Pitymys and its 

 associates and Microtus and its closest allies, have descended. 

 In external appearance most of the living species of Phenacomys 

 do not differ much from typical voles; they possess moderately 

 large eyes, well-developed ears, normal hands and feet, and a 

 moderately long, well-clothed tail. Two or three forms, some- 

 times placed in a special subgenus Arborimus, have, however, 

 remarkably long tails, and they retain the arboreal habits which 

 may have characterized the long-tailed ancestor of all voles. 



The skull resembles that of the more primitive species of 

 Microtus in general form, having a moderately long rostrum, 



