Comparisons of bonnevillei with the type and type series of Thomomys bottae wahwahensis show them to be of approximately the same size, but to differ as follows: Color: Slightly darker above and lighter below; postauricular patches smaller and lighter. Skull: Larger in every measurement taken, except breadth of rostrum which is smaller; skull not as flat; tympanic bullae more inflated ventrally; nasals and rostrum longer; extension of premaxillae posterior to nasals greater; interparietal smaller and more triangular; zygomatic arches more bowed out laterally; jugals heavier; interpterygoid space more widely V-shaped; upper incisors less massive.
The characters that distinguish bonnevillei from Thomomys bottae albicaudatus are: Size smaller. Color: Markedly lighter throughout. Skull: Shorter and wider; mastoid and zygomatic breadths greater; rostrum narrower but shorter; angle between rostrum and zygomatic processes of maxillae less; interparietal smaller and more triangular; extension of premaxillae posterior to nasals greater; upper incisors shorter, narrower and more recurved.
T. b. bonnevillei is indistinguishable in color from Thomomys bottae convexus, but differs from it in the following features: Size larger in nearly every measurement taken. Skull: Flattened dorsally as opposed to convex; zygomatic arches longer and weaker; jugals more nearly perpendicular; tympanic bullae larger; upper incisors longer; alveolar length of upper molar series the same, but molars narrower; rostrum longer but nasals shorter; extension of premaxillae posterior to nasals greater.
Topotypes of bonnevillei can be distinguished from those of both Thomomys bottae tivius and stansburyi by being larger in every measurement taken, by markedly lighter color throughout, and by ridged, massive, angular skulls rather than smooth, weak, nonangular skulls.
The races closest geographically to bonnevillei are Thomomys bottae robustus and T. b. sevieri. Compared with topotypes of robustus, bonnevillei differs in: Size larger. Color: Lighter throughout. Skull: Larger, although not as compact; zygomatic arches more widely spreading; jugals lighter; lacrimal processes not as prominent; zygomatic processes of maxillae not as robust; nasals more flared distally; extension of premaxillae posterior to nasals greater; alveolar length of upper molar series longer; molars larger; upper incisors longer, wider and darker in color; when placed ventral side down on a surface, the dorsal face of a skull of robustus is approximately parallel to the surface, whereas one of bonnevillei dips down in the occipital region.
T. b. sevieri can be easily distinguished from bonnevillei by being smaller in every measurement taken, darker in color, and by small, weak, smooth skulls as opposed to large, robust, ridged skulls.
Remarks.—Fish Springs, where bonnevillei occurs is a marshy area south of the barren, salt-desert country of western Utah. The source of water is springs at the base of the north end of the Fish Springs Mountains. Only the moist area supports pocket gophers. Specimens from Trout Creek, Juab County, twenty-five miles to the southwest are intergrades between bonnevillei and aureiventris, and are referred to the latter subspecies. The country between Fish Springs and Trout Creek in 1937 and 1940 lacked pocket gophers; it was of the playa and sand type. Probably T. b. bonnevillei was derived from T. b. aureiventris, a western mainland form of Pleistocene Lake Bonneville, through isolation and subsequent differentiation morphologically. The moist soils at Cane Springs, seven miles south of Fish Springs, had no pocket gophers when visited in 1940.
Specimens examined.—Total, 11, from the type locality.
Thomomys bottae centralis Hall
Thomomys perpallidus centralis Hall, Univ. California Publ. Zoöl., 32:445, July 8, 1930.