1914] Grinnell: Mammals and Birds of the Colorado VaUey 231 



In the bottomlands a few miles below Palo Verde, cotton rats were 

 found in apparent abundance March 31 to April 3. They were here 

 inhabiting a tule patch at the edge of a slough and a dense patch of 

 seedling willows adjacent. There were no runways in evidence, but 

 a dozen or more little piles of cut willow twigs were found lying on 

 the ground one to four feet from the edge of the water. 



The twigs were from four to eight inches in length with teeth-cuts 

 at each end, and there were from half a dozen to twenty of these 

 twigs in a pile. A few bits of tule stems were found in one place, and 

 in another a rather thick green tule stem was nearly eaten through. 

 The stomach of one of the rats taken contained a finely masticated 

 mass of material judged to be tule stem. 



Five miles northeast of Yuma, Sigmodon was found April 30 to 

 May 3 inhabiting a tract of wire-grass bordering a back-water slough 

 near the river, and flanked by young willow growth. Near Pilot Knob, 

 May 10, specimens were caught in thickets of cane surrounded by 

 dense arrowweed. 



A half-grown young one was caught on the last named date. A 

 gravid female was caught on April 30. 



The weight of one of the largest males taken (no. 10626) was 

 eighi ounces; that of a smaller male (no. 10632), though cpiite adult, 

 41/0 ounces. A very great range in size is also shown from the accom- 

 panying table of measurements, which lists all of the specimens taken 

 by the expedition. So great is the discrepancy between nos. 10626 

 and 10627, and other apparently fully adult males in our series, that 

 the existence of two distinct species was at first suspected. The two 

 large examples referred to present measurements far above any of the 

 specimens listed by Mearns (1907, p. 453), and some twenty per cent 

 above the typical size of the race eremicus as given by Bailey (1902, 

 p. 107). There are also apparent other differences concurrent with 

 the great size, namely, rather coarser pelage, greater hairiness of tail, 

 and a faintly pinkish cast to the coloration on the rump and sides. 



I sent specimens illustrating this state of affairs to Mr. Vernon 

 Bailey, who gives it as his opinion that the differences are due to 

 age; in this genus "old males often outgrow the bounds of specific 

 characters." In support of this contention, that the extremes are con- 

 specific, is the consideration of geographical probabilities : to-wit, that 

 experience teaches that it is not to be expected that two closely related 

 species in a genus exist together in exactly the same locality. 



