_Diagnosis._--Size medium (see measurements). Color: Upper parts and sides Pinkish Buff, purest on sides; underparts Pale Pinkish Cinnamon; inguinal and pectoral regions Pale Pinkish Buff; nearly all specimens have white on perineal region; nose grayish black; front feet, hind feet and distal third to half of tail white; postauricular patches black. Skull: Braincase moderately convex on dorsal surface; rostrum strongly depressed, giving the entire dorsal surface of the skull a "rocker-shape"; zygomatic arches heavy, short and widely spreading, widest posteriorly; upper incisors recurved, short and wide; molariform teeth large; alveolar length of upper molar series long; palatal pits deep; tympanic bullae moderately inflated ventrally; mastoidal breadth actually as well as relatively wide. _Comparisons._--Compared with topotypes of _Thomomys bottae wahwahensis_, _convexus_ is of approximately the same color, but differs as follows: Size smaller; tail, hind foot, and ear shorter. Skull: Rostrum longer, narrower and more depressed; skull convex rather than flat; nasals longer, and convex rather than flat; tympanic bullae larger; zygomatic arches shorter and more massive; molariform teeth larger. From topotypes of _Thomomys bottae centralis_, _convexus_ differs in: Size smaller; tail and hind foot shorter. Color: Uniformly lighter, more white in perineal region. Skull: Smaller, more convex; rostrum shorter, wider and more depressed; zygomatic arches shorter and heavier; mastoidal breadth actually, as well as relatively wider; tympanic bullae more inflated ventrally; upper incisors shorter and wider. Comparatively, topotypes of _convexus_ can be distinguished from those of _Thomomys bottae aureiventris_ by: Size smaller; tail and hind foot shorter. Color: Darker on upper parts; no "gold" on underparts. Skull: Smaller and more nearly flat; rostrum shorter and more depressed; zygomatic arches shorter, heavier and widest posteriorly rather than anteriorly; interpterygoid space V-shaped as opposed to lyre-shaped; upper incisors shorter, narrower and more recurved. Topotypical specimens of _convexus_ differ from those of _Thomomys bottae nesophilus_ as follows: Size smaller; tail and hind foot shorter. Color: Uniformly lighter throughout, Cinnamon Buff as opposed to Pinkish Buff. Skull: Smaller; rostrum heavier, shorter and more depressed; zygomatic arches shorter, heavier and not so widely spreading; no widening of supraoccipital as in _nesophilus_; upper incisors shorter and more recurved. When compared with topotypes of _Thomomys bottae albicaudatus_, _convexus_ shows the following differences: Size smaller; tail and hind foot shorter. Color: Markedly lighter throughout. Skull: Smaller, more convex and compact; rostrum shorter, heavier, more depressed and compact; zygomatic arches shorter and more robust; upper incisors shorter and more recurved.