126 Proceedings of the Biological Society of Washington. 



the State soon to be issued by the Colorado Geological Survey, 

 a proof sheet of which had been furnished to us. The map will 

 also aid in comprehending the geological history of the region 

 which has led to the isolation of groups of plants and animals 

 of restricted altitudinal range. 



Owing to seasonal differences in pelage colors, as well as dif- 

 ferences due to age, the diagnosis of closely related forms of 

 this genus is difficult and unsatisfactory unless one has access 

 to numerous specimens, representing various localities and 

 various seasons. 



In the University of Colorado Museum are two specimens 

 taken at Trappers' Lake by Mr. A. H. Felger, of Denver, 

 August 31, 1909, and three more in his collection, taken the 

 same day at the same place. All are females and apparently all 

 young, though full grown. Trappers' Lake is about nine or ten 

 miles south of Pagoda Peak, and the two localities are connected 

 by an unbroken mountain divide which should be just as favor- 

 able to these animals as either of the special localities mentioned, 

 so far as one may judge by observing the divide from the valley, 

 without actually traversing it. However, the description of 

 figginsi does not exactly fit the Trappers' Lake specimens. This 

 is likely due to difference in season, as the figginsi type was 

 taken on October 30, two months later than the Trappers' Lake 

 specimens, the former thus representing the winter pelage, 

 while the latter represents the worn summer pelage. There is 

 also considerable variation in the colors of the Trappers' Lake 

 specimens themselves, due possibly to difference in age, though 

 that is doubtful. Dr. Allen's comparison was based upon 

 winter pelage. 



We have compared these specimens with four from the Front 

 Range, taken near timber line, west and southwest of Boulder, 

 one on July 21, two on July 24, and one on August 26, hence 

 all representing summer pelage. Individually some specimens 

 from the two localities are scarcely distinguishable; collectively, 

 with the two lots placed in separate rows, side by side, the dif- 

 ference is clear. This, as is well known, is often true of various 

 forms of Peromyscus. Assuming that the Trappers' Lake lot are 

 true figginsi, and that the Front Range lot are true saxatilis, the 

 figginsi lot as a whole is considerably darker above than the 

 saxatilis lot. This is owing to a larger proportion of black and 



