82 MICROTIN^ 



been different, fossorial habits moulding what is essentially one 

 and the same primitive stock have in the end produced two 

 totally different results. Prometheomys digs its quite shallow 

 but extensive burrows with its hands and throws out the earth 

 in small heaps like a mole ; its powerful incisors are used chiefly 

 for the purpose of cutting roots which may obstruct its progress, 

 and for cutting the roots of the subalpine grasses upon which it 

 feeds. The hands, feet, and fur are the organs therefore chiefly 

 affected by the fossorial habits of Prometheomys; these habits 

 have not directly influenced either its skull or its teeth to any great 

 extent, and the chief cranial modifications of the genus are 

 correlated with the peculiar development of the temporal muscles 

 and therefore with the peculiarities of the food. Ellohius, on the 

 other hand, uses its incisor teeth and skull as a powerful shovel 

 and drill, in the manner of Spalax; accordingly the head and 

 fur are highly specialized in this genus, but the hands and feet 

 remain comparatively unmodified. 



In Ellobius the fur is very fine, short, dense, and mole-like; 

 the eyes are small ; the ears are reduced to a mere fold of naked 

 integument surrounding the external meatus and completely 

 hidden in the fur. The hands and feet are of moderate size, 

 armed with very small claws, and with naked palms and soles ; there 

 are five palmar and six plantar pads as usual, the plantar pads 

 being all moderately developed or small. The lateral borders of 

 the hands and feet are fringed with stiff hairs, those on the outer 

 margins being especially well developed. The tail is very short, 

 and is fully clothed with stiff hairs, which form a long thin 

 terminal pencil. 



The skull shows extreme fossorial specialization, being cuneate 

 in profile, with straightened and far-protruding incisors, extremely 

 shallow and slender rostrum, depressed anterior nares, flattened 

 dorsal surface, and slightly inclined occiput. The zygomata are 

 moderately stout and diverge posteriorly. The orbito-temporal 

 vacuities are rather small ; the interorbital region is short and wide, 

 about as broad as the rostrum. The temporal ridges are closely 

 approximated throughout, tending to fuse or in old age fusing 

 to form a sagittal crest extending from the interorbital region 

 backwards to the occiput as in Prometheomys. The interparietal 

 is very small, its sutures obliterated in old age. The squamosals 

 are large but do not show any tendency to encroach upon the 

 frontals anteriorly; their post-orbital crests are distinct, but 

 the shoulders of the braincase fall away from the interorbital 

 region much less abruptly than in Prometheomys and normal voles. 

 The infraorbital canal is modified; its upper portion for the 

 transmission of the infraorbital portion of the masseter medialis 

 muscle is somewhat enlarged, and the lower slit-like portion 

 normally serving for the transmission of the nerves and vessels 

 has disappeared. The rostrum in ventral view is long and 

 slender ; the anterior palatal foramina are very small. The palate 



