THE GROUND SQUIRRELS OP CALIFORNIA. 669 



and two distinct pelages separated by these. The spring molt occurs during April 

 (March 2S to May 1 according to specimens at hand), and the fall molt probably 

 during October though there are no specimens available to show its extent. The 

 remnants of the winter pelage during the spring molt become faded in some specimens 

 to a dull yellowish tone. This molt advances in a general way from the front back- 

 ward, but specimens often show a patchy or mixed coat on the back and rump. 



Measurements. — Average and extreme measurements, in millimeters, of seventeen 

 adult specimens from the Colorado and Imperial valleys are as follows : seven males : 

 total length, 247 (225-261) ; tail vertebrae, 95 (S5-107) ; hind foot, 36 (34-37) ; 

 greatest length of skull, 37.1 (35.2-38.2) ; zygomatic breadth, 23.5 (22.5-24.4) ; 

 interorbital width, 8.9 (8.3-9.5). Ten females: total length, 241 (216-258); tail 

 vertebrae, 91 (75-102) ; hind foot, 36 (33-38) ; greatest length of skull, 36.0 

 (34.3-3S.2) ; zygomatic breadth, 22.2 (21.3-23.4) ; interorbital width, 8.6 (8.0-9.5). 



Males are seen to be slightly larger than females. The ears in this species are 

 small, the rims rising not more than 3 millimeters (i inch) above their inner base. 

 In but few specimens did the collector attempt to secure the measurement of the ear. 



Weights. — Stephens (MS) found two females and a male to weigh together 12 

 ounces, an average of 4 ounces each. "All were thin." 



Type locality. — Fort Yuma [Imperial County], California (Baird, 1857, pp. 

 315-316). 



Distribution area. — Low-lying sandy areas on the Colorado and Mohave deserts. 

 Life-zone, Lower Sonoran (see fig. 23). More specifically: the Imperial Valley west 

 as far as La Puerta (Mus. Vert. Zool.) in extreme eastern San Diego County, north 

 to the southern end of Salton Sea, and east to old Fort Yuma ; thence north along 

 the Colorado River nearly or quite to the Nevada line ; and from the vicinity of 

 Needles and Blythe, iu the Colorado Valley, northwestward across the central part of 

 the Mohave Desert to at least as far as Kramer (Grinnell, MS), in west-central 

 San Bernardino County. Altitudes of occurrence, from 200 feet below sea-level to 

 2,300 feet above. The range of this species is not continuous over the area just 

 indicated (see fig. IS), but consists of many colonies more or less distantly isolated 

 from one another. 



Specimens examined. — A total of 28 from the following localities in California: 

 San Bernardino County, one-half mile north of Barstow, 1 ; Daggett, 1 ; Blythe 

 Junction, 4 ; Needles, 3. Imperial County : south end of Salton Lake, 6 ; six miles 

 south of Holtville, 2 ; Coyote Well, 4 ; Pilot Knob, 4 ; Colorado River opposite Cibola, 

 2. San Diego County : La Puerta, 1. 



The Yuma Round-tailed Ground Squirrel was first made known to 

 science in 1857 from specimens taken by an army officer stationed at 

 old Fort Yuma, which was situated on the California side of the Colo- 

 rado River opposite the present town of Yuma. It inhabits the hottest 

 of our southeastern desert valleys. Its metropolis lies in the Imperial 

 Valley and thence north along the valley of the Colorado ; a few colonies 

 occur also on suitable parts of the Mohave Desert. Over this general 

 region tlie species is by no means continuously distributed. It seems to 

 be very particular in its requirements, only level ground of a sandy 

 nature being as a rule inhabited at all. Places are preferred where 

 wind-drifted sand has been accumulated into mounds about the bases 

 of mesquite, creasote bushes, or salt bushes. Here the burrows are to 

 be seen opening up among the stems of the partly buried shrubs; and 

 the animals, if not actually seen themselves, are shown by their tracks 

 in the sand surface to be in the habit of foraging out across the bare 

 intervals for the seeds which are to be found sifted among the sand 

 particles. 



The squirrels themselves are usually shy and by reason of their 

 obscure coloration and especially the shimmering glare on the desert 

 surface are not readily observed unless particularly sought for. Neither 

 are their total numbers very great even where conditions are fairly 



