The Jordan River bisects Salt Lake County from north to south. Pocket gophers were taken at nine places east of the river, and at three places west of it.
Gophers from Salt Lake City and environs (east of the river) vary in color from almost black to dark cinnamon. Specimens from Draper, which locality is likewise east of the river, are uniformly lighter, but also vary in color. The skulls of animals from both localities are indistinguishable from each other and closely resemble those of topotypes. Specimens from the west side of the river, from Riverton, two miles west of Murray and Rose Canyon, Oquirrh Mountains, all are lighter in color than topotypes. The color varies from darkest at the north at Murray to lightest at the south at Riverton. This is exactly the reverse of what would be expected since Riverton is the locality geographically nearest to the type locality, Provo. The skulls are quite uniform and are all referable to albicaudatus. The Jordan River may be one factor which causes this lack of uniformity between the animals from the two sides of the river. Davis (1939:56-57) states that rivers are not barriers to movement of pocket gophers where the river completely freezes over and has the ice covered with thick snow. Although the Jordan River does occasionally freeze over, it is never frozen for more than a few days at a time, and snow in this area does not last for long periods. The material at hand indicates that the gophers from both sides of the river are referable to the same subspecies albicaudatus. The animals from the east side of the river are in the aggregate of characters the most typical of albicaudatus of any in the entire range. Those from the west side of the river, although definitely referable to albicaudatus do show some intergradation with Thomomys bottae robustus, the subspecies to the west.
The specimens from Bauer, Tooele County, are relatively uniform in color, and are considerably lighter than topotypes of albicaudatus. Their upper parts vary from Sepia to Saccardo's Umber as compared with near (13''''n) Black of the topotypes. The sides and underparts are lighter, due primarily to much less black in the underfur. They average slightly longer in total length, but shorter in hind foot. All cranial measurements are slightly smaller than in topotypes of albicaudatus. The shape of the skull closely resembles that of albicaudatus, although the rostrum, nasals, upper incisors and posterior tongues of the premaxillae tend to be narrower. This narrowness indicates intergradation with Thomomys bottae stansburyi, the race nearest to the west. These animals are in the majority of characters referable to albicaudatus.
Bauer is situated in extreme western Tooele Valley at the foot of Stockton Bar, a low pass between the Stansbury and the Oquirrh mountains. This valley lies to the west of the aforementioned Jordan River. Although these gophers are definitely referable to albicaudatus they are more unlike topotypes than are the animals from Riverton.
The specimens from Settlement Canyon, Oquirrh Mountains, Tooele County, show the same characteristics as those from Bauer.
In a large series of animals from St. John, in Rush Valley, Tooele County, the upper parts vary from black, even darker than topotypes of albicaudatus, to Tawny Olive, and the underparts vary from black through Cinnamon Buff to Pinkish Buff. Most of the animals are Cinnamon Buff. Although variable they approach albicaudatus in color. The total length, tail and hind foot of males are longer than in topotypes of albicaudatus; females differ in the same direction but only slightly. In both sexes the zygomatic breadth is less, but the mastoid breadth is greater than in albicaudatus. In size and shape of the lacrimal processes, and the great thickening of the jugal at the maxillo-jugal suture they approach robustus. They are much larger, however, and in the majority of characters are referable to albicaudatus.
What has just been said relative to the animals from St. John applies also to those from Clover Creek in the Onaqui Mountains of Tooele County. At the latter locality the tendencies towards robustus are accentuated. This is to be expected, since this locality is midway between St. John and the type locality of robustus. All characters considered, these animals are all referable to albicaudatus.
The animals from Little Valley, Sheeprock Mountains, Tooele County, resemble albicaudatus in color. They vary on the upper parts from near (1) Sepia to Clay Color, and ventrally from nearly black to Pinkish Buff. They are markedly smaller in every measurement taken, except zygomatic and mastoidal breadths, and extension of premaxillae posterior to nasals. This relatively greater breadth indicates intergradation with robustus to the west. These gophers are smaller in most measurements than any other population referred to albicaudatus. This is understandable because gophers from mountains usually are smaller and have weaker, smoother skulls than animals from low lands. Although approaching robustus in size and in some aforementioned cranial details, the aggregate of characters including color, make these animals referable to albicaudatus.
The animals from Fairfield, Utah County, are closer geographically to the type locality of albicaudatus than any other series, but morphologically are the least like topotypes. At first glance one is struck with the differences. They are uniformly Clay Color above, with Cinnamon Buff sides and flanks and Pinkish Buff underparts. Their color closely approaches that of robustus to the west which has Cinnamon Buff on the upper parts. Examination of eleven measurements of males and the same number for females, shows that the animals are nearest to robustus in two measurements, to albicaudatus in 12, distinct in 7 and intermediate in one. The general appearance of the skull is intermediate between that of the two above mentioned forms. The differences from albicaudatus in size and color may be correlated with the differences in soil at Fairfield and Provo. At Fairfield the soil is light-colored clay, but at Provo it is sandy and darker. Although they are intergrades between robustus and albicaudatus, the animals are referred to the latter race. Utah Lake and its outlet, the Jordan River, make a partial barrier between populations at Fairfield and at the type locality at Provo. During Pleistocene times, when Lake Bonneville was present it formed a complete barrier. Enough time has evidently elapsed since the disappearance of this lake to allow albicaudatus, the mainland form, to expand its range to the west. Intergradation has taken place, with the result that the animals from Fairfield, although unstable, agree with the mainland form, albicaudatus, in a majority of their characters.