1914] GrinncU: MammaJs and Birds of the Colorado Valley 243 



however, impossible to assort the series into two subspecies ou any 

 satisfactory basis, and, since the average appears nearest D. m. mer- 

 riami. all are placed under this name. 



Perognathus bombycinus Osgood 

 Yuma Pocket ilouse 



Twenty-five specimens were procured (uos. 9956-9977, 1074:2- 

 10744), three being preserved as alcoholics, and the rest as skins- 

 with-skulls. Only two localities are represented, near Ehrenberg on 

 the Arizona side of the river, where eighteen specimens were caught 

 March 25 to 30, and near Pilot Knob on the California side of the 

 river, wliere seven were taken May 7 to 15. Those at Ehrenljerg were 

 caught on areas of wind-blown sand, especially where heaped about 

 the bases of creosote bushes, on the mesa a mile or more back from the 

 river. At Pilot Knob the species was found ou aeolian sand accumula- 

 tions and in shallow sandy washes on the desert mesa. 



In all cases Perognathus bombycinus was found on common ground 

 with Dipodomys deserti, Dipodomys merriami, Peromyscus eremicus, 

 and Perognathus penicillatus. The five rodents named thus have verj' 

 similar associational preferences. While Perognathus bombycinus was 

 not found by us as near the river as the second bottom (as were all the 

 other rodents named) it is fair to assume that it may so occur where 

 conditions favor. And like the others of the same association, transfer 

 of individuals from side to side is likely to have taken place at inter- 

 vals in the past. This might be advanced as a reason for the close 

 similarity of the representations of bombycinus on the two sides of 

 the river. 



The original description of Perognathus bombycinus (Osgood, 1907, 

 pp. 19, 20) was based on a single specimen from Yuma, Arizona, and 

 two from ju.st over the Mexican line in Sonora. Our present series 

 bears out to the dot the cranial characters assigned by Osgood, namely, 

 as compared with Perognathus panamintinus bangsi, enorniovLsly 

 inflated audital bullae and mastoids, the latter conspicuously pro- 

 tuberant posteriorly, and narrow interparietal. Externally our speci- 

 mens of bombycinus are distinguishable from bangsi in their very pale 

 coloration, which consists in lighter ground color (dilute pinki.sh buff) 

 and ahnost obsolete black tippings to the hairs dorsally. Although, 

 as compared with bangsi, bombycinus has greatly enlarged audital 

 bullae, the external ear is not of appreciably greater size. 



