1915.1 



THOMOMYS PERPALLIDUS GROUP. 



69 



Distribution. — Colorado Desert, southern California, from White- 

 water south to Salton Sea (fig. 6). 



Characters. — Size medium, hind foot 31 mm. in adult male topo- 

 type; tail long; ears small; color pale buffy gray or whitish; mammae 

 ill 4 pairs, inguinal 2-2, pectoral 2-2. 



Color. — -Summer pelage: Upperparts buffy or cream color; ear 

 patches dusky; nose and cheeks grayish brown; underparts, feet, and 

 tail whitish, thinly haired. Winter pelage: Paler than in sunimer, 

 upperparts creamy or whitish. Young, buffy gray. 



Sicull. — Rather light and slender, without conspicuous ridges or 

 angles; zygomatic arches slender and parallel; nasals rather short, 

 cuneate, truncate, or slightly emarginate at posterior tips; inter- 

 parietal small and irregularly triangular, oval, or quadrate; bullae 

 short but full and rounded. Dentition rather light ; incisors abruptly 

 decurved. 



Measurements. — Average of 5 topotypes ( c? ad.) : Total length, 241 ; 

 tail vertebrae, 84; hind foot, 31.5. Average of 4 topotypes ( 9 ad.): 215, 

 78, 30. Sicull (of topotype, ad.): Basal length, 37; nasals, 15; 

 zygomatic breadth, 26; mastoid breadth, 21; interorbital breadth, 

 6.4; alveolar length of upper molar series, 7.7. 



Remarlis. — ^This very pale, desert species occupies the scattered 

 moist oases in the hottest and driest of our deserts. Apparently its 

 range is not continuous, as no gopher hills are to be found over wide 

 areas of bare hot desert. The numerous colonies are to some extent 

 isolated, and differences too slight for even subspecific recognition 

 mark the specimens from almost every locality, but the main char- 

 acters hold true over a wide area. Specimens from Carrizo Creek 

 grade toward the more robust form alhatus of the Colorado River 

 bottoms on the California side, west of Yuma. A small series from 

 Salt Creek, north of the Salton Sea, seem to go better with perpallidus 

 than with any of the other three forms to which they might almost 

 as well be referred. Direct intergradation is shown with perpes and 

 through it with griseus and aureus. 



The aureus group as heretofore considered now becomes the 

 perpallidus group and includes perpallidus, alhatus, cJirysonotus, 

 perpes, canus, aureus, apacJie, cahezonse, operarius, and probably also 

 latirostris, cervinus, and sinalose. 



Specimens examined. — Total number, 68, as follows: 



California: Agua Dulce, 1; Baregas Spring (8 miles east), 5; Colorado Desert 

 (no specific locality), 5; Fish Spring, 2; Mission Creek, 1; Palm Springs, 35; 

 Salt Creek (Riverside County), 8; Salton Sea (west side), 1; Wliitewater, 10. 



