HYPERACRIUS 333 



sals, frontals, and parietals correspondingly and progressively 

 more or less modified with advancing age. Posteriorly the 

 temporal ridges are also rather closely approximated in adults 

 and the intertemporal portions of the frontals, parietals, and 

 interparietals are correspondingly and progressively diminished. 

 Auditory bulla? very small, somewhat flattened, thin-walled, and 

 without any internal filling of spongy bone. Rostrum long and 

 shallow. Anterior palatal foramina small ; short and narrow. 

 Upper incisors projecting well in advance of the nasal tips. 

 Mandible with m^, displaced lingually by the shaft of the lower 

 incisor, which ascends into the condylar process, where its termina- 

 tion is marked by a hump on the outer surface of the bone; 

 angular processes of mandible rather small. 



Cheek-teeth light, rootless, and tall-crowned, the capsule 

 of m^ rising up conspicuously in the floor of the sphenorbital 

 fissure. Enamel well differentiated as in Microtus. Re-entrant 

 folds without visible cement. Enamel patterns of teeth essentially 

 as in Alticola, but 7n^ with its posterior loop shorter, broader, 

 and more reduced than usual in the latter genus (Fig. 95). 



Remarks. — Hitherto Hyperacrius has been accorded merely 

 subgeneric rank within the genus Microtus; but the peculiar 

 combination of cranial and external characters shown by all 

 its species clearly entitles the group to be regarded as a distinct 

 genus. As already stated in the introduction Hyperacrius seems 

 to be an offshoot from some primitive species of Alticola; it 

 has made considerable progress along a path of fossorial special- 

 ization parallel apparently to that pursued by Ellobius or its 

 forerunners. 



Four forms are at present known, and their distinguishing 

 characters may be keyed as follows : — 



A. Size large (hind-foot about 18 mm.; condylo-basal length of 



skull 28-29 mm.). 



o. Fur highly modified and mole-like; colour variable, upper 

 parts light brown or dusky. Skull less specialized than in 

 the smaller species ; interstephanic portion of frontals less 

 reduced ; zygomata more normal . . H. wynnei Blanford. 



b. Fur short and dense, not specially modified ; colour of upper 

 parts yellowish-brown. Tail obscurely bicoloured. Skull 

 imperfectly known H. aitchisoni Miller. 



B. Size smaller. (Hind-foot not exceeding 17 mm.; condylo- 



basal length of skull not exceeding 25 mm.) Fur short and 

 dense, not specially modified; colour dask reddish- brown, 

 near sepia. Tail obscurely bicoloured. Skull more 

 highly specialized ; interstephanic portion of frontals more 

 reduced ; zygomata unusually elevated. . . //. fertilis True. 

 a. Ear larger (meatus to tip 9-11 mm.), visible above the fur. 



H. f. fertilis True. 

 6. Ear smaller (meatus to tip 7-9 mm.), concealed in the fur. 



H. /. hrachelix Miller. 



