728 RODENTIA 



middle, narrowing a little posteriorly to width of interparietal, 

 and more noticeably in front, finally disappearing in region 

 between postorbital processes, where the ridges come together ; 

 each ridge with a distinct angle at middle and usually with 

 another less evident angle about half-way between middle and 

 point of juncture ; interparietal large, somewhat variable in 

 form, usually ligulate with obliquely truncate outer extremities 

 and slight anterior median projection, but sometimes almost a 

 perfect parallelogram about twice as wide as long ; lamb'doid 

 crest high and conspicuous at sides, but reduced to a mere 

 angular edge along posterior border of interparietal ; truncation 

 of occiput somewhat oblique, slightly more so than in Microtus 

 agrestis, the entire occipital condyle usually, though not always, 

 visible when skull is viewed from above ; outline of occiput 

 viewed from behind elliptical, flattened at the middle, slightly 

 more than half as high as wide ; paroccipital processes large, 

 widely spreading, their extremities not applied to bullae, their 

 bases continued upward as well-developed ridges joining lamb- 

 doid crest near outer extremities of interparietal ; foramen 

 magnum higher than wide, its upper border narrowly rounded or 

 forming a distinctly pointed arch ; base of brain-case with no 

 special features, the basioccipital with ill-defined median longi- 

 tudinal ridge, the width of the bone along antei'ior border 

 contained about 2£ times in its median length ; auditory bullae 

 rather large (about 9 mm. in antero-posterior diameter), but not 

 highly inflated, their ventral border not extending to level of 

 cutting surface of molars. Interorbital region deeply constricted 

 posteriorly, where in old individuals the width is less than half 

 that in lachrymal region ; ridges early uniting to form an 

 evident median crest which, however, does not extend forward 

 beyond the deeply constricted region. Zygomata heavy, rather 

 gradually spreading, the greatest zygomatic breath at glenoid 

 level ; vertical expansion evident but not abrupt. Rostrum of 

 the same relative size as in Microtus agrestis, but somewhat more 

 slender anteriorly ; nasals distinctly smaller relatively to rostrum 

 than in M. agrestis, their anterior termination falling decidedly 

 short of level of gnathion, their greatest combined breadth 

 noticeably less than that of rostrum in same region ; posterior 

 border of nasals squarely truncate at level of about middle of 

 zygomatic root, ascending branches of premaxillaries reaching 

 somewhat further back ; incisive foramina relatively shorter 

 than in Microtus agrestis, their posterior border falling short of 

 alveolar level by 1 mm. or more, their anterior extremity lying 

 decidedly nearer maxillo-premaxillary suture than to posterior 

 surface of incisor. Bony palate with no special peculiarities as 

 compared with that of Microtus agrestis ; lateral pits large, but 

 rather shallow ; median ridge tending to be low and not sharply 

 defined ; mesopterygoid space with anterior border varying from 

 squarely truncate to pointedly arched ; posterior half of ptery- 



