and two distinct pelages separated by these. The spring molt occurs during April (March 28 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 backward, 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 vertebræ, 95 (85–107); 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 vertebræ, 91 (75–102); hind foot, 36 (33–38); greatest length of skull, 36.0 (34.3–38.2); 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 (? 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, in 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. 18), 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 Colorado 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 the 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
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