244 University of California Publications in Zoology [Vol. 12 



The measurements of twenty adult males from our series are, aver- 

 ages and extremes: total length 139mm. (130-149), tail vertebrae 79 

 (70-85), hind foot 18.5 (17-19.5). 



I am unable to find any differences between the specimens from 

 Ehrenberg and those from Pilot Knob, the species being thus identical 

 on the two sides of the river. We found no traces of pocket mice of 

 this group anywhere else along the river. Osgood (1907, p. 20), how- 

 ever, records typical bang si from Needles, California. It is quite pos- 

 sible that there is a hiatus between the ranges of bangsi and bomby- 

 cinus. At any rate, among the large series of bangsi in the Museum 

 from the Mohave desert and from the west side of the Colorado desert 

 from San Gorgonio Pass to the Mexican line, there is no individual 

 showing close enough approach in characters to bombycinus to war- 

 rant classing it as an intermediate. Although the affinities of bomby- 

 cinus are clearly with the panamintinus group, there is good evidence 

 for carrying the former as a full species. 



Curiously, only one out of the entire twenty-five bombycinus taken 

 was a female. This, and the fact that no young-of-the-year were found, 

 would seem to show that up to May 15 the breeding season had not 

 yet begun. For experience with other rodents in different places 

 indicates that as soon as the young are born the females forage abroad 

 actively, and are then caught in at least equal proportions to the males. 



Perognathus formosus Merriam 

 Long-tailed Pocket Mouse 



A series of 44 specimens was obtained, 40 skins-with-skulls (nos. 

 9652-9691) and 4 alcoholics (nos. 10794-10797). These were taken 

 exclusively on the California side of the river and represent localities 

 as follows: five miles below Needles, 6; opposite The Needles, 12; 

 Chemehuevis Valley, 8 ; Riverside Mountain, 5 ; near Blythe, 1 ; Pot- 

 holes, 5 ; Pilot Knob, 7. 



The known range of this species is thus carried southeast along 

 the west side of the Colorado River to the Mexican line (see Osgood, 

 1900, pp. 40, 41). The distribution is not, however, continuous, for 

 marked restriction is shown to hilly country and rough mesas. The 

 immediate valley of the Colorado is avoided, as also the broad, flat and 

 low desert depressions between the mountain ranges. 



Yet within the rough country trapping showed that Perognathus 

 formosus occurred most frequently upon the narrow strips of loose, 



