From _B. sawrockensis_, _B. brachygnathus_ differs in: molar row slightly longer; teeth slightly less depressed; masseteric ridge extends farther anteriorly; incisors more prooedont. From _B. musculus_, _B. brachygnathus_ differs in: jaw smaller; molar row slightly shorter; molars less depressed; incisors slender, shorter, narrower, and more prooedont. From _B. taylori_, _B. brachygnathus_ differs in: incisor more slender, shorter, more prooedont; diastema shorter. _Remarks._--The molar teeth of _B. brachygnathus_, although worn, resemble those of _B. taylori_ more than those of any known fossil species. Gidley (1922:124) stated that the absence of the divided anterior lobe of the first molar (anterior median fold) in _brachygnathus_ was one of the chief characters separating _brachygnathus_ from _taylori_. In _taylori_, the anterior median fold characteristically is only slightly developed, and in some specimens is absent. _B. brachygnathus_ differs from _taylori_ chiefly in prooedont incisors, which feature seems to preclude _brachygnathus_ being ancestral to _taylori_. _B. brachygnathus_ may have been a specialized divergence from _B. minimus_. =Baiomys minimus= (Gidley) _Peromyscus minimus_ Gidley, U. S. Geol. Surv. Prof. Papers, 131:124, March 15, 1922. _Baiomys minimus_, Hibbard, Amer. Midland Nat., 26:352, September, 1941; Gazin, Prof. U. S. Nat. Mus., 92(3155):488, 1942. _P. [eromyscus] minimus_, Wilson, Carnegie Inst. Washington Publ., 473:33, May 21, 1936. _Type._--No. 10500, U. S. Nat. Mus.; left mandibular ramus bearing m1-m3 and incisor; 2 mi. S Benson, sec. 22, T. 17 S, R. 20 E, Late Pliocene (Blancan, Gazin, 1942:482), Cochise County, Arizona. _Referred material._--None. _Diagnosis._--Ramus small for the genus; molar teeth depressed; cingular ridges (ectolophids) of m1 and m2 well-developed; anterior median fold present (appearing larger owing to chip of enamel missing); external reentrant fold of m3 progresses half way across crown of tooth; diastema short; incisor moderately large, recurved; length of molar row, 2.6 mm. _Comparisons._--For comparisons with _B. brachygnathus_, _B. kolbi_, and _B. sawrockensis_, see accounts of those species. From _B. rexroadi_, _B. minimus_ differs in: anterior median fold deeper; incisor longer, more recurved, less prooedont; molars slightly more depressed (though worn). From _B. musculus_, _B. minimus_ differs in: over-all size of jaw and molars smaller; incisors shorter; masseteric ridge more depressed. From _B. taylori_, _B. minimus_ differs in: anterior median fold slightly deeper; molar teeth more depressed; cingular ridges on m1 and m2 better developed; masseteric ridge more depressed.