68 MICROTIN^ 



palate; the temporal ridges fuse anteriorly in old age. 

 The auditory bullae are densely spongy within, and their mastoid 

 portions are noticeably inflated. The persistently growing 

 cheek-teeth are substantially as in Microius; but m^ has only 

 three salient angles on each side, and m.^ has the third outer 

 angle obsolete; ?% is of characteristic form, with a posterior 

 loop, five closed triangles, and a small anterior loop of peculiar 

 squarish shape. 



Lagurus, widely distributed in Eastern Europe, Asia, and 

 Western North America, is a remarkably isolated, in some 

 respects a very primitive, in others a highly specialized genus. It 

 appears on the whole to be most closely related to Microtus 

 and its nearer allies, and no doubt traces its descent from some 

 primitive Phenacomys -like vole. The external form is highly 

 modified for fossorial habits, the general appearance of the 

 animal being very Lemming-like. The fur is long and very 

 soft. The eyes are moderate. The ears, without an antitragus, 

 are very small and hidden in the fur. The hands and feet are short 

 and broad ; the thumb bears a small pointed nail ; the claws 

 are of moderate length, the hind ones very slightly the longer; 

 there are five plantar tubercles, but they are completely con- 

 cealed beneath the dense hairy covering of the soles. The tail is 

 very short and is densely clothed, the hair forming a short terminal 

 pencil. The mammary formula is 2^2 = 8. 



The short and broad skull has a remarkable superficial 

 resemblance to that of Dicrostonyx in dorsal view. The rostrum 

 is moderately short and broad, and the zygomata leave its sides 

 squarely, without any prezygomatic notch. The interorbital, 

 region is moderately constricted, but the temporal ridges, although 

 very salient in adults, are rather widely separated anteriorly by 

 a deep median sulcus much as in Dicrostonyx; the ridges are 

 rather closely approximated behind, compressing the interparietal, 

 which is abruptly truncated laterally. The squamosals are 

 widely separated anteriorly, with rather small but prominent 

 peg-like post-orbital processes. The palate is essentially as in 

 Microtus, with a long postero-median sloping septum. The 

 pterygoid fossse are short and deep, their floors lying at a 

 level considerably dorsal to the ventral surface of the basi- 

 sphenoid. The auditory bullse are very large and are highly 

 modified, with the external meatus shortly tubular and much 

 straitened; the cavity is partly filled with a dense sponge of 

 bone ; the mastoid portion and tegrnen tympani are similarly 

 enlarged and spongy, the mastoid part bulging outwards rather 

 conspicuously between the paroccipital process and the lateral 

 process of the supraoccipital ; the stapedial artery is enclosed in 

 a bony tube. The mandible is normal, although the angular 

 process is rather small ; the lower incisor displaces m.^ as usual. 

 The upper incisors are strongly curved or slightly " opisthodont." 



The cheek-teeth are persistently growing and exceedingly 



