1922] Kellogg: 8y)wpsis of Microtus Mordax 287 



First lower molar with posterior border of transverse loop unevenly 

 convex, two inner and two outer closed sections and with internal 

 reentrant angles much deeper than external ones. The closed triangles 

 designated as paraconid and metaconid, larger than protoconid. 



Second lower molar with three transverse sections or loops, the 

 internal reentrant angles much deeper and narrower than external; 

 posterior loop but slightly if at all larger than adjoining loop, the 

 enamel border enclosing the paraconid considerably produced so that 

 tip lies in same plane as tips of metaconid and entostylid. 



Remarks. — ^The writer has not examined the type specimen of 

 Microtus mordax, though a topotype series of eleven specimens, 

 loaned for study by the Biological Survey", was critically studied. 

 These specimens agree in all essential details with specimens of this 

 subspecies from the Modoc region of California, and from the Pine 

 Forest Mountains of northern Nevada. 



As originally described, Microtus mordax was a wide ranging form, 

 with frequent interruptions or gaps in its distribution, but with sup- 

 posedly little variation throughout its range. The accumulation of 

 a considerable series of specimens in the Museum of Vertebrate 

 Zoology from many points within California has enabled the writer 

 to better elucidate the relationships of the various local varieties which 

 make up this composite. Microtus mordax mordax, as now restricted, 

 occupies within the state only the Modoc or Great Basin region. 

 Unfortunately no specimens have been collected in the Pit River region 

 east of Bear Creek, and as the specimens from the latter locality have 

 not been examined, the area of intergradation between mordax and 

 sierrae remains to be determined. A few specimens from Parker 

 Creek exhibit certain characters which indicate that this entire Modoc 

 region may be one of intergradation, though the series as a whole 

 average smaller both with respect to skulls and to external measure- 

 ments. There is often considerable uncertainty as to the exact dis- 

 position of individual specimens from intermediate areas, and this is 

 certainly true of various specimens from the Modoc Region. The 

 number of specimens of Microtus mordax tnordax available for study 

 is much too small to permit accurate comparison with the material 

 of sierrae, though it does indicate that the latter form is a much 

 more robust and heavily proportioned meadow mouse than typical 

 mordax. 



