44 MICROTIN^ 



are more primitive than those of Evotomys ; in most of the species 

 cusp n is largely developed in 7n^ and nv^, in which teeth it forms 

 a large extra postero-internal salient angle. 



Anteliomys is a closely related genus which goes a little 

 further than Eothenomys in three respects : — the temporal ridges 

 tend to fuse in the interorbital region, the palate develops a 

 conspicuous though horizontal median spinous process poste- 

 riorly, and cusp 11 is more reduced in vi^ and m- than is usual 

 in Eothenomys. In one respect it is more primitive; m^ is 

 much more complex, having five or even six salient angles on 

 each side. The first outer fold of this tooth is usually shallow, 

 leaving the first outer triangle confluent with the anterior loop — 

 a character seen in other genera, such as AUicola. 



Alticola is also an offshoot from some primitive form^f 

 Evotomys, as is clearly shown by the structure of the palate and 

 the general form of the molars. The cheek-teeth have become 

 rootless, and m^ is noticeably displaced by the incisor. The 

 molars are characterized by the great width of the re-entrant 

 folds, which contain very little cement, and by rather perfect 

 alternation of the inner and outer elements ; these features give 

 the teeth in most species a peculiar long-drawn-out appearance. 

 In pattern m}, irfi and all the lower molars are about as reduced 

 as in normal Evotomys; irfi shows from species to species an 

 interesting series of gradations, beginning with forms in which 

 the tooth is about as complex as in Anteliomys, and ending with 

 others in which there are only three outer and two inner salient 

 angles ; the first outer fold of this tooth is, as already mentioned, 

 almost invariably shallow. The skull is lightly built and in 

 essential respects closely resembles that of Evotomys. The 

 temporal ridges are widely separated in the interorbital region; 

 the post-orbital squamosal crests are moderately developed; 

 in the palate the lateral bridges are frequently incomplete 

 and the hinder edge is often furnished with a blunt median spine. 

 The auditory bulte are simple, thin walled, and sometimes greatly 

 inflated; they are destitute of spongy tissue within, and the 

 stapedial artery is naked. The species are more or less highly 

 specialized for life at high altitudes; the more primitive, with 

 relatively complex teeth, normal Evotomys-\We essential external 

 characters, including long, thinly-clothed tails and naked soles, 

 occur at lower elevations; the more specialized forms, with 

 simplified teeth, short and densely clothed tails, and hairy soles, 

 inhabit higher regions. Some remarkable species, inhabiting 

 the bare talus slopes of Central Asia, have acquired remarkably 

 flattened skulls fitting them for life in rock crevices ; these have 

 been referred to a special subgenus Platycranius by Kascenko, 

 but apart from the peculiar flattening of the skull there is nothing 

 to distinguish them from the more specialized forms of Alticola. 



Hyperacrius is apparently a peculiar descendant from 

 some form of AUicola. The genus has become specialized for 



