240 University of California Publications in Zoology ' [Vol.12 



creosote association. Thus the two species might exist on the same side 

 of the river, with complementary ranges. Still, as far as known, 

 neither species occurs on the opposite side of the river, the river thus 

 serving as the factor of absolute delimitation in the respective cases. 

 It is probable that the two species had latterly a separate course of 

 origin, having been derived from distantly located stocks. 



Dipodomys deserti deserti Stephens 

 Big Desert Kangaroo Rat 



The thirty-nine specimens secured were preserved as follows : nos. 

 10352-10381, skins with skulls; nos. 10708, 10700. complete skeletons; 

 nos. 10724-10730, alcoholics. The following localities are represented : 

 Arizona side : Mellen, 23 ; Ehrenberg, 2 ; twenty-five miles below Ehren- 

 berg, 1 ; five miles northeast of Laguna, 3. California side : Riverside 

 Mountain. 1 ; near Pilot Knob. 9. 



This rat shows strong preferences for the aeolian sand association. 

 It was found present practically wherever such accumulations reached 

 an extent of an acre or more of sufficient depth to contain the burrows. 

 This essential depth was seen to be at least a foot. Because of the 

 limited powers of digging, as shown both by weak incisors and small 

 front feet and claws, only such loose and fine-grained substratum could 

 be occupied by this rodent. The packed floor of the mesa or the hard- 

 pan of the hillsides was uninhabitable. 



The presence of this Dipodomys could always readily be recognized 

 by means of conspicuous burrows, the mouths of which were usually 

 at all times open. The looseness of the ground in which the burrows 

 are dug results in their frequent caving in when men or horses walk 

 over them. When the wind had not acted to efface them, the tracks 

 of the rats could be plainly seen on the surrounding sand surface 

 (pi. 11, figs. 16, 17). These tracks showed imprints of the hind feet 

 and tails, indicating the tripedal mode of ambulation characterizing 

 the Heteromyidae. 



The associational restriction of this large kangaroo rat gives the 

 impression that it possesses a colonial habit. But it was observed that 

 where proper conditions were continuous, as at the north base of 

 Pilot Knob, the local distribution over the suitable area was practically 

 uninterrupted. The burrows occur in small groups, each group prob- 

 ably representing but a single home center and occupied at times by 

 a single adult, at others by an adult pair, and at others by adult and 



