191-*] Grinnell: Mammal* and Birds of the Colorado Valley 249 



Colorado River bottom. The presence of the species was ascertained 

 at every station where appropriate ground was trapped, from Mellen 

 nearly to the Laguna dam. 



Among all the specimens taken, only two were juvenals. These 

 were taken above Laguna on April 24 and 25. None of the numerous 

 females taken earlier in the season contained embryos, so that the 

 breeding season would appear to be deferred in the case of this 

 species until the advent of hot weather. 



Perognathus spinatus spinatus Merriam 



Spiny Pocket Mouse 



This pocket mouse was found only on the California side of the 

 river. A series of 125 specimens was taken, nos. 9536-9651 (skins- 

 with-skulls) , 10713. 10714 (skeletons). 10787-10793 (alcoholics). 

 Localities are represented as follows : opposite The Needles (practically 

 topotypes of the species). 14; Chemehuevis Valley, 1; Riverside Moun- 

 tain, 26 ; opposite Cibola, 36 ; twenty miles above Picacho, 32 ; eight 

 miles east of Picacho, 6 ; Potholes. 4 ; Pilot Knob, 6. 



The spiny pocket mouse proved to have much the same associational 

 preferments as the antelope ground squirrel, namely, the hills and 

 rough-surfaced mesas of the desert. It was found close to the river 

 only where the riparian bottomland associations were pinched in to 

 merest traces of their elements by the abutment of the hills. Places 

 of this nature, where spinatus was caught within as near as a hundred 

 yards of the water's edge, and yet on ground perfectly appropriate 

 to the species, were opposite The Needles and twenty miles above 

 Picacho. 



At the latter locality the only exception to the above statements 

 came to notice. An adult female was caught among the willows on 

 first bottom and within seventy-five feet of the water, hence where 

 proper conditions of topography and rising water might have resulted 

 in transporting the animal to the opposite side of the river. But the 

 excessive rarity of such a combination of circumstances probably 

 accounts for the fact that Perognathus spinatus has never been found 

 to the east of the Colorado River. The strip of bottomland at the 

 point where this wanderer was captured was only about three hundred 

 yards wide, and the species was found to occur commonly on the 

 hillsides down to the outer edge. The individual probably occurred 

 merely as a forager. 



