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, proödont; 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 proödont.