131-*] GrinneU: Mammals and Birds of th( Colorado Valley 235 



miles northeast Laguna, 6; near Yuma, 14. Two adult males from 

 twenty miles above Picacho and near Yuma weighed nine ounces each; 

 an adult female from the former station weighed seven ounces. 



No differences are observable between the specimens from the two 

 sides of the river. The river is evidently in scarcely any degree a 

 barrier to distribution in this species. Rather has the Colorado bottom 

 served as a highway of invasion for the species from its center of distri- 

 bution, which is manifestly to the southward, the desert on either side 

 acting as confining walls. An examination of the series of specimens 

 shows an apparent slight decrease in size up the river, that is, away 

 from the assumed center of dispersal. But since there is also con- 

 tinued, though diminished, growth of individuals with age, and because 

 of the few examples from any one of the uppermost stations, this 

 direction in variation up the river is not to be considered as established. 

 Much more material is needed. 



Neotoma intermedia desertorum Merriam 

 Desert Wood Rat 



The 41 specimens taken (nos. 10424-10462, 10716, 10717) indicate 

 stations of occurrence as follows : California side : opposite The 

 Needles, 3 specimens; Riverside Mountain, 2; opposite Cibola, 1; 

 twenty miles above Picacho, 13 ; eight miles below Picacho. 3 ; Potholes, 

 2; Pilot Knob, 11. Arizona side: foot of The Needles, 3; above Bill 

 Williams River, 1 ; ten miles below Cibola, 2. 



None of our specimens was trapped within the riparian strips; 

 all were taken on the desert proper, though at The Needles, where 

 because of the abrupt rock walls the riparian element is in places 

 reduced to a mere nearly vertical band, desert wood-rats or signs of 

 them were found within a few yards of the river on the opposite sides. 



Here scanty accumulations of sticks were observed in crevices 

 among loose boulders on hillsides or in clefts of the walls of ravines. 

 At Riverside Mountain, some two miles back from the bottomlands, 

 nests of large size were noted among boulders; this was true, too, at 

 Pilot Knob, where also sign was seen and the rats themselves caught 

 around clumps of desert tea far out on the mesa. 



Fifteen of the specimens taken at the different stations are young 

 of varying sizes. Two very small young of 172 and 167 mm. length, 

 respectively, were trapped March 6 and 7 at The Needles, indicating 

 early breeding. Others nearly as small were taken at Pilot Knob, May 



