1914] Grinnell: Mammals and Birds of the Culm-ado Valley 247 



lish the point, trapped on dry surfaces, indicating a preference for 

 dryness. On thoroughly dried-out, rather high sand banks along the 

 river where the bottom strip was narrow, sign was noted to within a 

 few feet of the edge of the swift current. Individuals were trapped 

 on sand bars, at the time high and dry, but so slightly elevated that 

 rising water would at first form islands of them and then engulf them 

 completely. 



No doubt, as with Peromyscus, aggressive individuals of Perog- 

 nathus penicillatus are often thus caught and set adrift, thus giving 

 a chance for transfer from one side of the stream to the other. As 

 discussed in a general way elsewhere, there seems to be significant cor- 

 relation between the fact of this free invasion of the river bottom 

 and existence and similarity of the representatives of the species on 

 both sides of the river. 



The breeding season of penicillatus is of greater duration than that 

 of the other pocket mice of the region, as shown by the following data. 

 A female taken April 19 contained five embryos; another taken May 4 

 contained four embryos. Gray-coated young-of-the-year were taken as 

 early as March 5 and at intervals from then on through April and 

 into May. 



After study of the Colorado River series in connection with the 

 120 additional specimens in the Museum from various localities in 

 the Salton Sink region of the Colorado desert, I have come to the 

 conclusion that the recognition of the name angustirostris as apply- 

 ing to any of the Colorado River specimens is impracticable. Osgood 

 (1900. pp. 45—47) described a race angustirostris from the Colorado 

 desert, type locality Carriso Creek, west side of the desert. The 

 Museum has a good topotype series, and also a series from Mecca, in 

 the bed of the Salton Sink at the northwest end of Salton Sea] 



The characters assigned to "angustirostris" were, as compared 

 with penicillatus, small size, less massive skull, and longer and more 

 slender rostrum. Osgood referred his few specimens from the Colorado 

 River, from Ehrenberg and above, to penicillatus, while those from 

 Fort Yuma and all localities to the west he listed under angustirostris. 



Close scrutiny of the extensive material now at hand shows great 

 instability in all the alleged characters. There is as notable fluctuation 

 in size from place to place up and down the river, as across the Colorado 

 desert. The topotypes of angustirostris include some skulls as massive 

 as many from Mellen, Arizona. 



