757 



rounded off behind, not obliquely truncate, the occipital condyles 

 barely visible when skull is viewed from above ; paroccipital 

 processes slender and inconspicuous, rather closely applied to 

 surface of bullae, their tips descending below lip of foramen 

 magnum, the ridges extending upward from their bases slightly 

 developed, much obscured by the somewhat unusual inflation of 

 petrous portion of squamosal ; floor of brain-case nearly smooth, 

 but with slightly indicated median ridge ; diameter of basioccipital 

 along suture contained about two and one-half times in median 

 length ; auditory bullae moderately large, about as in Microtus 

 arvalis, though somewhat less evenly inflated, their anterior 

 borders encroaching more deeply into ectopterygoid pits. Inter- 

 orbital region short, abruptly though not deeply constricted, the 

 least width usually greater than that of rostrum, the dorsal 

 surface nearly flat, .its outer edges becoming slightly angular in 

 old age though never developing distinct ridges. Zygomata 

 weak, scarcely expanded at middle, 

 rather abruptly though not widely 

 spreading, the two arches parallel 

 through a considerable portion of their 

 extent ; deflection of arches slight, 

 the median portion lying about at level 

 of middle of skull ; anteorbital foramen 

 relatively smaller than in Microtus 

 arvalis. Rostrum relatively weak, its 

 least depth behind incisors about equal 

 to width at same region ; nasals rather 

 noticeably narrowed behind middle, 

 their posterior border squarely trun- 

 cate at about level of middle of zygo- 

 matic root, the nasal branches of 

 premaxillaries extending a little further 



backward ; incisive foramina scarcely as large as in Microtus 

 arvalis, their posterior extremity at level of front of to 1 , their 

 anterior extremity separated from incisors by distance equal to 

 about twice their greatest combined breadth. Palate relatively 

 somewhat wider than in Microtus arvalis, its surface less sharply 

 sculptured ; grooves moderately well-defined, but lateral pits 

 usually rather shallow, their width slightly exceeded by the 

 short, flattened median ridge; pterygoids nearly parallel, the 

 hamulars slightly turned outward ; mesopterygoid space rounded 

 anteriorly ; ectopterygoid pits relatively smaller than in Microtus 

 arvalis. Mandible slender, with weak articular process, its 

 general form essentially as in M. arvalis. 



Teeth. — Upper incisors strongly curved, the roots forming 

 slight protuberances in infraorbital foramina, the anterior portion 

 nearly perpendicular, so that anterior surface is barely visible 

 when skull is viewed from above ; cross section of shaft obscurely 

 triangular, the anterior border longest, slightly concave near 



KQ. 155. 

 Pitymys subterraneus. Nat. size. 



