756 



RODENTIA 



Microtus arvalis, the tail about one-third as long as head and 

 body, the feet slender and without special modification for 

 burrowing, the incisors not protruding conspicuously from mouth. 

 Ear small, but scarcely more so than in Microtus arvali.s, its 

 form with no special peculiarities except that meatal lobe is 

 slightly less developed ; both surfaces of ear conch essentially 

 naked at base, thinly clothed with rather long hairs distally. 

 Eye relatively smaller than in Microtus arvnlis. Muzzle not 

 peculiar. Front foot as in Microtus arvalis, except that the claws 

 ai'e slightly larger. Hind foot slender, its general form as in 

 M. arvalis, the sole similarly hairy behind tubercles, the tubercles 

 of the same form and relative size, their only peculiarity the 

 complete absence of the sixth. Tail as in M. arvalis. Mammaj : 

 4, all inguinal. 



Colour. — Upper parts usually a dark nearly uniform hair- 

 brown, tinged with sepia along middle of back, everywhere 

 inconspicuously varied by silvery reflections and blackish hair- 

 tips ; ground colour sometimes more nearly approaching mars- 

 brown or wood-brown, but rarely if ever with any distinct buffy 

 cast ; basal portion of hairs slate-black ; underparts pale smoke- 

 grey, occasionally with a slight wash of broccoli-brown, the slaty 

 bases of the hairs everywhere showing through at surface ; feet 

 an indefinite drab or dusky grey, often tinged with dark hair- 

 brown ; tail dull whitish grey beneath, dark brown above, the 

 two colours sometimes though not usually well contrasted. 



Skull. — The skull is smooth and lightly built, with large, 

 somewhat flattened brain-case, rather wide interorbital region, 

 and small rostrum. Dorsal profile nearly straight from near 

 posterior extremity of nasals to a little behind middle of parietals, 

 then slightly convex to posterior margin of interjjarietal, beyond 

 which the occiput curves abruptly downward ; nasals sloping 

 forward at an angle of about 11° ; ventral profile nearly straight 

 from about middle of rostrum to lower border of auditory bullae, 

 the general outline of skull as viewed from the side rather 

 narrowly cuneate. Brain-case long as compared with that jjortion 

 of the skull which lies in front of it, the distance from back 

 of interorbital constriction to hinder border of interparietal 

 decidedly greater than breadth over roots of zygomata, its out- 

 line rather broadly oval, its surface smooth, or at most with 

 slightly indicated ridges along outer sides of parietals ; lambdoid 

 crests slightly developed at sides ; postorbital ridges low and 

 indistinct, rarely if ever developing an angular projection ; ''■' 

 interparietal narrowly ligulate, the outer extremities usually 

 pointed, the anterior border with median projection ; in lateral 

 view the brain-case appears much flattened, its depth through 

 auditory bulke about 65 jjer cent of occipital breadth ; occiput 



* A character that ofteu proves convenient in distinguishing between 

 imperfect sliulls of this species and the smaller forms of Microtus, in which 

 the ridges are usually more developed and somewhat angular. 



