Thomomys bottae nesophilus Durrant, Bull. Univ. Utah, 27 (No. 2):2, October, 1936; Marshall, Journ. Mamm., 21:156, May 14, 1940.
Type.—Male, adult, skin and skull, No. 1136, Museum of Zoölogy, University of Utah; Antelope Island, Great Salt Lake, Davis County, Utah; April 20, 1935; collected by S. D. Durrant; original number 761.
Range.—Known only from the type locality.
Diagnosis.—Size medium (see measurements); claws on front feet long. Color: Upper parts Cinnamon Buff; lighter below; sides Pinkish Buff interspersed with gray; pectoral and inguinal regions Cinnamon; nose grayish black; postauricular patches black. Skull: Interparietal wedge-shaped; tympanic bullae small; dorsal surface of lambdoidal prominence 3 mm. wide rather than developed as a crest; jugals nearly straight; zygomatic arches strongly rectangular.
Comparisons.—Compared with topotypes of Thomomys bottae albicaudatus, nesophilus is of approximately the same size, but differs as follows: Claws on front feet longer. Color: Lighter throughout; tail white terminally, but much darker at base; postauricular patches smaller. Skull: Interparietal wedge-shaped as opposed to roughly quadrangular; lambdoidal eminence more of a crest than a ridge; tympanic bullae smaller; jugals more nearly straight; zygomatic arches more nearly rectangular.
From topotypes of Thomomys bottae aureiventris, nesophilus differs in: Size smaller; claws on front feet longer. Color: Darker throughout; postauricular patches larger. Skull: Heavier, more massive; zygomatic arches more robust and convex laterally rather than concave; interparietal wedge-shaped rather than roughly quadrangular; braincase more nearly flat; tympanic bullae markedly smaller; upper molariform series longer; molariform teeth wider and heavier; interpterygoid space V-shaped rather than lyre-shaped.
The race nearest geographically to nesophilus is T. b. minimus from Stansbury Island, Great Salt Lake. It can easily be distinguished from minimus by the following features: Size much larger; claws on front feet longer and thicker. Color: Darker throughout; postauricular patches larger and with more admixture of buff colored hairs. Skull: Larger in every measurement taken; wide and robust as opposed to narrow and slender; zygomatic arches more widely spreading and angular; braincase more nearly flat; tympanic bullae actually larger, but relatively smaller; lambdoidal eminence flat-topped rather than a crest; interparietal wedge-shaped as opposed to quadrangular; teeth larger.
Remarks.—The affinities of nesophilus of Antelope Island are unquestionably with albicaudatus of the eastern and southern mainland. At the time of this writing (1945), Antelope Island is not truly an island, but only the tip of a broad peninsula projecting westward into Great Salt Lake. Nevertheless, the area of occurrence of nesophilus is effectively isolated by the exposed, sandy lake bottom that is unsuited to occupancy by pocket gophers. Fluctuations in the level of the Great Salt Lake have broken and reëstablished this connection with the mainland many times. Each of the several other kinds of mammals which are known from both the island and the mainland show no differentiation on the island. These are kinds (see Marshall, 1940:156), which more freely cross the exposed, sandy lake bottom. I, myself, have noted tracks of coyotes going to and from the island. The pocket gopher, nesophilus, so far as known is the only mammal which has developed a subspecies endemic to the island. The beach levels of Pleistocene Lake Bonneville are well marked on both Antelope Island and Stansbury Island, which is fifteen miles west of Antelope Island. On the eastern side of Antelope Island the lower beach levels of this prehistoric lake are farmed. Although sought for elsewhere on this island, pocket gophers were found only in the farmed land. On Stansbury Island there has been no farming, and the endemic pocket gophers, minimus, although sought for elsewhere on that island were found only above the highest beach levels of the ancient lake. Evidently these pocket gophers still occupy only that part of Stansbury Island that projected above water during the greatest height of Lake Bonneville. Farming on Antelope Island may have developed a more favorable environment for pocket gophers, thus causing them to move down to the lower levels from that part of the island that was above water during Pleistocene times.
Specimens examined.—Total, 5, from the type locality.