From B. sawrockensis, B. brachygnathus differs in: molar row slightly longer; teeth slightly less depressed; masseteric ridge extends farther anteriorly; incisors more proödont.
From B. musculus, B. brachygnathus differs in: jaw smaller; molar row slightly shorter; molars less depressed; incisors slender, shorter, narrower, and more proödont.
From B. taylori, B. brachygnathus differs in: incisor more slender, shorter, more proödont; 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 proödont incisors, which feature seems to preclude brachygnathus being ancestral to taylori. B. brachygnathus may have been a specialized divergence from B. minimus.
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.