1914] Grinnell: Mammals and Birds of the Colorado Valley 233 



On the other hand, these aberrant examples might be recognized 

 as mutants, that is, variants of phylogenetic significance though not 

 geographically isolated. After going over the ground, I lean per- 

 sonally towards the notion of the existence of an exceptionally wide 

 range in ordinary fluctuational variation, as accounting for the 

 extremes in question. This is coupled with the inadequacy of the 

 material available, so that the distinctions are emphasized. With large 

 series more nearly normal frequency would be expected. 



In this connection it may be pointed out that an alleged race, 

 Sigmodon hispidus arizonae Mearns (see Bailey, 1902, p. 108), has 

 been described from Fort Verde, Yavapai County, Arizona, with char- 

 acters close to those appertaining to the Colorado River giants in 

 question. In size these two specimens from the Colorado equal or 

 exceed that of the type of arizonae, so that the existence of a separate 

 race based on size is questionable. 



Reithrodontomys megalotis deserti J. A. Allen 

 Desert Harvest Mouse 



This rodent was confined strictly to the riparian strips. Although 

 not found by us above Ehrenberg, it probably does occur in suitable 

 environs as far up the Colorado as these are afforded, that is, up to 

 the steep-walled canons above Fort Mohave. Specimens were trapped 

 close to the water's edge, usually in grassy places adjacent to tracts 

 of willows. In some cases these mice were on islands cut off by chan- 

 nels from connection with the shore. In practically all cases the 

 ground they occupied would at high water be entirely submerged, so 

 that the mice would either have to swim to higher ground or take 

 refuge in the willows and drift piles. Doubtless they swim freely and 

 are also swept back and forth across the river channels at flood time. 



Twenty-one specimens were preserved (nos. 10165-10185). The 

 localities of capture were: Arizona side: Ehrenberg, five miles north 

 of Laguna, and Yuma; California side: near Palo Verde, and five 

 miles northeast of Yuma. Young over half grown were taken May 2, 

 and on the same day a female containing five embryos. 



Neotoma albigula venusta True 

 Colorado Valley Wood Rat 

 This species of wood-rat was in distribution the exact associational 

 complement of Neotoma intermedia desertorum : it was restricted to 



