_Diagnosis._--Ramus of medium size to large for the genus; lower incisor short, narrow transversely, prooedont; anterior median fold of m1 reduced or absent; cingular ridges of m1 and m2 moderately well-developed; m3 large relative to m1 and m2; average and extreme measurements of lower molars of seven specimens are, 3.0 (3.0-3.1). _Comparisons._--For comparisons with _B. sawrockensis_ and _B. rexroadi_, see accounts of those species. From _B. brachygnathus_, _B. kolbi_ differs in: molar row longer; m3 and jaw larger; diastema longer; masseteric ridge not so far forward; molars more depressed. From _B. minimus_, _B. kolbi_ differs in: molar row longer; m3 larger; jaw larger; diastema not so acutely curved; incisor shorter, narrower transversely, more prooedont. From _B. musculus_, _B. kolbi_ differs in: anterior median fold of m1 slightly developed or absent, instead of well-developed; m3 larger (not reduced), external reentrant valley broad and extending farther across crown of tooth; incisor smaller, and more prooedont; cingular ridges of m1 and m2 less well-developed. From _B. taylori_, _B. kolbi_ differs in: molars larger, more depressed; incisor shorter, more prooedont; m3 smaller relative to m1 and m2; external reentrant valley of m3 broad, extending farther across crown of tooth. _Remarks._--The slight development or absence of the anterior median fold in _kolbi_ suggests that it was specialized. The anterior median fold is well-developed in all species of _Baiomys_ save _B. brachygnathus_ and _B. taylori_, in which the fold is only slightly developed or absent. _B. kolbi_ may have paralleled _B. taylori_ in specialization for a diet of grasses and for a life in open country. =Baiomys brachygnathus= (Gidley) _Peromyscus brachygnathus_ Gidley, U. S. Geol. Surv. Prof. Papers, 131:124, March 15, 1922. _Baiomys brachygnathus_, Hibbard, Amer. Midland Nat., 26:352, September, 1941. _P. [eromyscus] brachygnathus_, Wilson, Carnegie Inst. Washington Publ., 473:33, May 21, 1936. _Type._--No. 10501, U. S. Nat. Mus.; right mandibular ramus bearing m1-m3, and incisor; 2 mi. NE Curtis Ranch house, near a line between sec. 28 and 29, T. 18 S, R. 21 E, Mid-Pleistocene (Hibbard, 1958:25), Cochise County, Arizona. _Referred material._--None. _Diagnosis._--Ramus small for the genus; m3 reduced; jaw reduced anteroposteriorly; incisor short, slender, prooedont; cingular ridges well-developed, posterior ectolophid continuous from protoconid to hypoconid in m1 and m2; diastema short; length of molar row 2.8 mm. _Comparisons._--For comparisons with _B. rexroadi_ and _B. kolbi_, see accounts of those species. From _B. minimus_, _B. brachygnathus_ differs in: jaw not so slender anteriorly; masseteric ridge not so far anterior; cheek-teeth slightly broader, less depressed, therefore, more hypsodont; incisor shorter, more prooedont.