Page:Natural History of the Ground Squirrels of California.djvu/94

674 coat is so short and thin that any scars there may be in the skin show through as dark spots. These appear irregularly on certain specimens, usually those which examination of the teeth shows to be the older individuals.

Measurements.—Average and extreme measurements, in millimeters, of twenty adult specimens from Whitewater, Palm Springs and Mecca, in Riverside County, are as follows: Ten males: total length, 241 (220–251); tail vertebræ, 90 (79–97); hind foot, 36 (35–40); greatest length of skull, 36.6 (35.2–38.9); zygomatic breadth, 23.0 (21.6–24.6); interorbital width, 8.9 (8.0–9.7). Ten females: total length, 240 (229–264); tail vertebræ, 89 (80–102); hind foot, 35.7 (34–39); greatest length of skull, 36.1 (35.6–37.5); zygomatic breadth, 22.8 (22.0–23.4); interorbital width, 8.8 (8.6–9.4).

Type locality.—Palm Springs, Riverside Coimty, California (Elliot, 1903, p. 242).

Distribution area.—The northwestern arm of the Colorado Desert between Salton Sea and San Gorgonio Pass (see fig. 18). Life-zone, Lower Sonoran. More specifically, the Coachella Valley, entirely within Riverside County, from Mecca northwest to Whitewater Station; altitude from -200 to 1,130 feet.

Specimens examined.—A total of 41 from the following localities, all in Riverside County: Palm Springs, 6; Whitewater Station, 18; Mecca, 17.

This race of Round-tailed Ground Squirrel was first made known from specimens collected in the vicinity of Palm Springs, out on the Colorado Desert near the northeast base of San Jacinto Peak. Subsequent exploration has shown it to be limited to the relatively small area of flat desert lying between Salton Sea and the upper part of San Gorgonio Pass, and shut in narrowly by the mountain walls on either side.

The slight features by which this subspecies is distinguishable from the Yuma Round-tailed Ground Squirrel of the Imperial Valley southeast of Salton Sea may be inferred to have arisen as a result of the action of the body of water which formerly filled the Salton Sink to sea level in cutting off or isolating the animals in the northwestern arm of the Colorado Desert and thus giving them a chance to develop peculiarities all their own. The ancient predecessor of the present Salton Sea is known to geologists as Blake Sea, and this inland sea extended from the base of the Chocolate Range of mountains on the northeast to the very foot of the Santa Rosa Mountains on the southwest, thus constituting an impassable barrier to any animal closely restricted, as is the Round-tailed Ground Squirrel, to dry, level, sandy ground. However this may have been, the Palm Springs subspecies now ranges down the Coachella Valley from the northwest nearly to the upper end of the present Salton Sea, in the vicinity of Mecca. One can imagine the animal life of the desert floor now retreating, now advancing, with the fluctuations of the old Blake Sea since the time it was first cut off from the Gulf of California by the slowly growing delta of the Colorado.

The general habits of the Palm Springs Round-tailed Ground Squirrel are probably closely similar to those of the Yuma and Death Valley animals. The first-named is fairly common locally, though it rarely forces itself on the attention; it has to be specially looked for. At Mecca in March and April (1908) one or two were caught nearly every day in oat-baited rat-traps set on sandy mounds beneath mesquites. Yet the animals themselves were rarely seen. On April 26 one was surprised up in a mesquite; upon being shot it was found to have parts of a mesquite flower in its mouth. The senior author was told that at the experimental date farm near Mecca these squirrels had been seen eating the dates.