48 MICROTIN^ 



Arvicola is thus shown to be a direct descendant of Mimomys ; 

 but the species now so widely spread over Europe and Asia are 

 probably not to be regarded as the immediate offspring of M. 

 intermedius and allied British species ; for certain dental reasons 

 it is more probable that they have come down from close 

 relations of M. intermedius which once existed in Eastern Europe 

 or Asia. The living members of the genus all show well-marked 

 fossorial specialization which tends to become extreme in members 

 of the A . scherman group ; in the more familiar riparian species 

 slight specializations fitting these voles for aquatic habits have 

 been superimposed upon fossorial characters. 



Modern s'peciesoi Arvicola are large voles with massive, strongly 

 ridged, and angular skulls when adult. The temporal ridges 

 fuse in the interorbital region, and the squamosals, with well- 

 developed post-orbital crests, approach each other, encroaching 

 upon the frontals anteriorly as age advances The palate 

 posteriorly is now essentially as in normal voles; the pterygoid 

 fossae are deep. The auditory bullae are rather small but 

 now show a slight development of spongy tissue within, and 

 the stapedial artery is completely enclosed in a bony tube. In 

 the more fossorial species the occiput is characteristically inclined, 

 pressed forwards above, and the upper incisors are noticeably 

 straightened and protruding, the animal evidently using the 

 skull and incisor teeth as one of its chief tools in digging. The 

 cheek-teeth are persistently growing, with normally differentiated 

 enamel, and with cement present in the re-entrant folds. The 

 enamel pattern is characterized by the simplicity of m^, with 

 only three salient angles on each side, and of m^ in which there 

 are only three closed triangles between the posterior and anterior 

 loops. In very young stages of wear the anterior loop of m^ 

 shows some extremely interesting ephemeral complications 

 which are described at p. 107. 



In this genus, as in many other members of the Microtinae, 

 persistent growth of the cheek-teeth appears to be accompanied 

 by persistent growth of the skeleton; in the oldest indi- 

 viduals examined, among the enormous amount of fossil and 

 recent material at my disposal, not only are the molars still in 

 vigorous growth but the epiphyses of the limb-bones are still 

 unfused with their shafts (Plates III and IV). Apparently, that is 

 so far as actual observation goes, voles of this genus are animals 

 that never stop growing and never grow old. But no doubt, 

 if one could keep the vole alive in natural conditions, but secure 

 from the fatal stroke of accident, a time would come when cheek- 

 teeth and skeleton would cease to grow and the animal would 

 become senile and die in the normal manner. 



Phaiomys is an interesting genus restricted to the highlands 

 of Central Asia, in which region it is usually found inhabiting the 

 damp meadows bordering the Alpine water-courses. It is closely 

 related to Arvicola, and seems like the latter to be descended 



