262 University of California PtibUoations in Zoology [Vol. 21 



First upper molar with anterior transverse loop prolonged into 

 narrow parastyle, its long axis in reverse direction to long axis of 

 transverse loop of Pm-. The enamel enclosing the section designated 

 as metaconule considerably constricted and prolonged. 



Second upper molar with an anterior transverse loop, its long 

 axis nearly parallel to long axis of anterior transverse loop of Pm-, 

 but with considerable variation in outline ; closed sections are smaller 

 than those of M^; enamel section enclosing metacone may or may not 

 be closed (figs. L, M) ; just posterior to metacone, the long terminal 

 loop is subtended externally by a slight notch, and internally by one 

 or two reentrant angles; this loop in its simplest form (fig. L) is 

 indented at middle by a deep reentrant angle which imparts to it a 

 strongly curved crescentic outline, the anterior metaconule longer than 

 secondary metaconule ; the posterior limb of crescent enclosing meta- 

 style may form an additional internal reentrant angle (fig. 0). 

 Triangle enclosing hypocone may fuse with that of paracone (fig. P). 



Fourth lower premolar with posterior transverse loop, normally 

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

 loop usually indented by deeper reentrant angle on inner side than 

 on outer. Inner side of tooth with four or five well developed re- 

 entrant angles; outer side of tooth with three deep reentrant angles 

 and a shallower fourth one. The large anterior loop designated as 

 secondary paraconid of variable outline, often forming an external 

 prolongation (fig. E). Reentrant angles separating enamel sections 

 enclosing secondary protoconulid, protoconulid, and protoconid, deeper 

 at base than reentrant angles that separate enamel sections enclos- 

 ing metaconid, mesostylid, and entoconid. Posterior loop narrowly 

 crescentic, with long axis slightly oblique with reference to that of 

 tooth row, the internal portion enclosing entostylid longer than that 

 designated as hypoconid. 



First lower molar with transverse posterior loop, two inner and 

 two outer closed sections, and with two inner and two outer deep 

 reentrant angles. The closed triangles designated as paraconid, proto- 

 conid, and metaconid of approximately the same size (in Microtus 

 mordax, paraconid and metaconid are larger than protoconid). 



Second lower molar with three transverse loops, each side with 

 two reentrant angles, the antero-external angle normally present; 

 posterior loop larger than either of the other sections, the internal 

 portion enclosing the entostylid curved forward at tip. Internal angle 

 of metaconid not deflected forward as in Microtus calif or nicus. 



Remarks. — The type of Arvicola montana was collected at the 

 headwaters of the Sacramento River and described by Titian R. Peale 

 (1848, pp. 44-45). Vernon Bailey (1900, p. 28) after careful com- 

 parison of various specimens from that general region decided that 

 the type specimen agreed in all essential details with specimens from 

 Sisson. Dr. T. S. Palmer in a conversation informed the present 

 writer that various statements in Peale 's journal indicate that Bailey's 

 selection of Sisson as the type locality is correct. 



