232 CALIFORNIA ACADEMY OF SCIENCES [Prog. 4th Ser. 



The shorter the pelage the greater is the tendency for the 

 phimbeous bases of the hairs ventrally to show through, 

 consequently differences in length of pelage are more readily 

 perceptible below than they are above. Applying this test to 

 our series of P. longicaudus it is noted that spring and summer 

 skins tend to have shorter pelage than those taken at other 

 times of year. 



No. 21143, collected April 20, has the pelage comparatively 

 thin. Ten specimens, taken October 24 and 25, are not 

 conspicuously different from those collected in July. 



There is a not readily tangible tendency toward paleness in 

 the summer skins. The slightly darker shade noted in the 

 winter examples is apparently due to the longer cinnamon 

 tipping rather than to any real difference in hue. 



No molt lines or other indications of molt, aside from the 

 slight difference in length of hair, are observable. Whether 

 there is a definite time of molt cannot be stated. It is not 

 improbable that there is a gradual hair renewal late in the fall, 

 perhaps during November, and that the "summer pelage" is 

 simply the worn winter pelage remaining over from this molt. 



IV Relations of Phenacomys albipcs to the two sub- 

 genera OF Phenacomys 



The complex of its characters relates Phenacomys albipes 

 more closely to P. longicaudus than to any other known form 

 of the genus, as was implied by Merriam in the original 

 description (1901, p. 125). For thirteen years the type 

 specimen of albipes remained unique, and it is only recently 

 that a second specimen has been collected (see Jewett, 1915, 

 pp. 37-38). 



Cranially Phenacomys albipes stands off by itself. Its skull, 

 as compared with that of oropliiliis, intermedius, and longi- 

 caudus, has an appearance of length and narrowness (see 

 Plate 15). Testing by actual measurement we find that there 

 is no clear dimensional difference in greatest length (see tables 

 of cranial measurements), albipes being exceeded by four of 

 the nine specimens of longicaudus measured, and by three of 

 the four specimens of orophihis measured. The zygomatic 

 arches, however, do not spread so widely in albipes as in 

 longicaudus, orophilus or intermedius. In fact, this dimen- 



