286 University of California Pvhlications in Zoology [Vol.21 



Mandible liglit and slender, similar to that of sicrrae. Ridge for 

 masseteres laterales well developed. Coronoid process long, its base 

 narrow, its extremity rising above level of condyle and cnrved back- 

 ward at tip. Angular process relatively stout, curved outward, but 

 with posterior border not extending as far as plane of condyle. Peri- 

 pheral swelling over base of incisor, Ijnng for the most part within 

 masseteric ridge. Base of mandibular foramen above or on a level 

 with cutting surface of last lower molar. Mental foramen on lateral 

 face situated below superior surface of diastema. 



Teeth. — Both upper and lower molariform series differ from those 

 of Microtis niontanus in a general tendency toward wider reentrant 

 angles, with apices of both salient and reentrant angles more rounded, 

 and with enamel section designated as protoconid smaller than either 

 paraconid or metaconid. Compared with those of Microtus calif orni- 

 cus the molariform series are slightly smaller; otherwise only certain 

 details of the enamel folding constitute the diagnostic differences. 

 Fourth upper premolar is longer than either the first or second molars 

 (fig. B). First upper molar with four closed sections. Fourth lower 

 premolar with five and occasionally six closed triangles. 



Fourth upper premolar normally with anterior transverse loop 

 and four alternating closed sections ; the external reentrant angles 

 deeper than the internal, their points extending across long axis of 

 the tooth ; anterior portion of transverse loop designated as proto- 

 conule somewhat flattened superiorlj^, and long axis of loop forms 

 an oblique angle with long axis of tooth. Triangles designated as 

 protocone and hypocone with apices more rounded than either para- 

 cone or metacone. Loop forming metaconule with no internal pro- 

 longation. 



First upper molar with anterior border of transverse loop irregular 

 in appearance, its long axis in reverse direction to long axis of 

 transverse loop of Pm-. Metaconule is not conspicuously lengthened 

 posteriorly. 



Second upper molar with long axis of anterior transverse loop 

 nearly parallel with long axis of anterior transverse loop of Pm-, but 

 normally with more convex anterior profile. The closed sections are 

 smaller than those of M- ; enamel section enclosing metacone normally 

 closed posteriorly (fig. C) though sometimes open superiorly (fig. E) ; 

 the terminal loop is subtended externally by notch at base of enamel 

 section designated as metacone and internally by one reentrant angle ; 

 this loop in its simplest form (fig. D) is indented at middle by deep 

 reentrant angle, though external outline is quite variable (fig. E), 

 seldom crescentic ; an incipient internal reentrant angle is occasionally 

 formed on posterior limb of loop designated as metastyle (fig. D). 

 Triangle enclosing hypocone may fuse with that of metacone (fig. E). 



Fourth lower premolar with posterior transverse loop, normally 

 with three inner and two outer closed triangles, and a large anterior 

 loop, usuallj^ with deeper internal reentrant angles than external 

 ones. Inner side of tooth with four or five well developed, reentrant 

 angles; outer side of tooth with three deep reentrant angles and a 

 shallower fourth one. Contour of large anterior loop designated as 

 secondary paraconid variable, rarely forming an external prolonga- 

 tion. Posterior loop narrowly crescentic, with oblique long axis. 



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