709 



blackish brown on nape, elsewhere a mere darkening of normal 

 colour. 



Skull. — The skull attains a greater length than that of 

 Microtus agrestis, but the zygomatic breadth and breadth of 

 brain-case scarcely if at all exceed the corresponding dimensions 

 in the smaller animal, so that the general form is conspicuouly 

 less robust. Dorsal, ventral and occipital profiles essentially 

 as in 31. agrestis, except that there is usually a slight concavity 

 in interorbital region, and the contrast between occipital and 

 rostral depths is less pronounced. Brain-case long and narrow, 

 slightly oval in outline, the moderately developed postorbital 

 processes not sufficiently prominent to cause the anterior portion 

 to appear scjuare ; upper surface rather strongly arched laterally, 

 not flattened as in 31. agrestis ; lateral ridges moderately 

 developed ; interpai'ietal relatively 

 small, its area distinctly less than 

 half that of parietal, its general 

 outline almost evenly biconvex except 

 for a slight median projection on 

 anterior border ; occiput when viewed 

 from behind essentially as in M. 

 agrestis, but more rounded above ; 

 floor of brain-case without special 

 peculiarities, but region of basal 

 suture depressed rather than ele- 

 vated (as in 31. agrestis) when skull 

 is held upside down ; auditory bulla 

 somewhat larger and more inflated 

 than in 31. agrestis, but not peculiar 

 in form. Interorljital region narrow 

 as in 31. agrestis, but more elongated. 

 Zygomata standing out from sides 

 of skull noticeably less than in 31. 

 agrestis and 31. arvalis ; median 

 expansion well defined, rather more 

 abrupt than usual ; infraorbital foramen wide, its lower border 

 less extended downward behind root of incisor than in 3Iicrotus 

 agrestis and 31. arvalis. Rostrum with no special peculiarities ; 

 nasals terminating posteriorly on level with nasal branches of 

 premaxillaries a little in front of middle of zygomatic root ; 

 incisive foramina shorter than in 31. agrestis, strongly contracted 

 posteriorly, terminating a little in front of level of alveolus of 

 m^ Bony palate with no special peculiarities, but lateral pits 

 rather small, and median sloping ridge broad and flat, somewhat 

 as in the sub-genus Chionomys. Mandible as in 31. agrestis, 

 but more slender. 



TfeiA.— Incisors essentially as in 3I{croius agrestis, but front 

 face of upper teeth less oblicjuely rounded off" at outer side. 

 Size of molars relatively to that of skull about as in 3Iicrotus 



Fia. 146. 



Microtus ratticeps. Nat. size. 



