628 RODENTIA 



from bullae, their points extending to slightly below level of 

 lower lip of foramen magnum. Floor of brain-case smooth 

 except for a median ridge on anterior half of basioccipital ; width 

 of basioccipital along anterior border about one-third median 

 length. Auditory bullae large, rounded and evenly inflated, their 

 surface smooth, their walls thin and translucent ; anteriorly they 

 overlap pterygoids for a distance of about 1 mm., though without 

 much encroaching on pterygoid fossa or developing a noticeable 

 beak. Interorbital region wide and smooth, its least breadth 

 conspicuously greater than that of rostrum at region of . greatest 

 width of nasals, its upper surface with broad and shallow but 

 noticeable longitudinal depression. Zygoma slender, scarcely 

 expanded at middle, the arches when viewed from above strongly 

 convex in front but spreading abruptly enough to bring the 

 greatest zygomatic breadth a little in front of middle of orbit ; 

 anterior zygomatic root with faint concavity ov6r infraorbital 

 foramen. Rostrum short and weak, tapering noticeably forward 

 when viewed either from above, below, or the side ; protuberances 

 over roots of incisors slight; nasals short, bluntly truncate 

 posteriorly at level a little in front of middle of zygomatic root, 

 usually somewhat exceeded by nasal branches of premaxillaries ; 

 outer border of nasal when viewed from above essentially 

 straight from posterior border to widest region. Incisive 

 foramina long and rather wide, extending from about 2 mm. 

 behind alveolus of incisor to within 1 mm. of level of m 1 . Palate 

 rather short and narrow, slightly concave laterally, its posterior 

 border, at level of anterior loop of m 3 , variable in form but 

 usually with a slight median convexity and a slight concavity at 

 each side ; mesopterygoid space long, nearly parallel-sided, the 

 hamular processes very slightly bent outward, applied against 

 inner surface of bullae. Mandible slender and weak, particularly 

 the hinder portion, into which the root of incisor penetrates so 

 slightly as to produce no evident thickening ; masseter ridge low 

 and inconspicuous except in old individuals ; coronoid process 

 rather long, noticeably curved backward, its extremity nearly 

 reaching level of condyle ; articular process long, slender and 

 compressed, the dental foramen lying below its base at level of 

 cutting surface of molars ; angular process slender and rather 

 long, convex externally, concave internally, its main axis curving 

 outward and upward. 



Teeth. — Compared with those of the European species of 

 Microtus the incisors are slender and weak, and the molars are 

 small, with both salient and re-entrant angles rounded and ill- 

 defined. Upper incisor projecting downward, the cross-section 

 of shaft bluntly triangular, the inner surface widest, the anterior 

 and postero-external surfaces narrower and sub-equal; enamel 

 well defined, extending over entire anterior surface and extreme 

 anterior portion of inner surface; cutting surface of tooth 

 distinctly hollowed but not sufficiently to produce any suggestion 



