Part 4

Type.--No. 4670, Univ. Kansas; left mandibular ramus bearing m1-m3, and incisor; Rexroad fauna, Locality no. 2, Upper Pliocene, Meade County, Kansas.

Referred material.--Univ. of Michigan Nos. 24840, 24851, 27493, 27496, 27501, 28862-28867.

Diagnosis.--Ramus medium in size for the genus; incisors small, prooedont; anterior median fold of m1 slight; cingulum of all molars poorly developed; average and external measurements of lower molar row of seven specimens are, 2.7 (2.6-3.0).

Comparisons.--For comparisons with B. sawrockensis and B. minimus, see accounts of those species. From B. kolbi, B. rexroadi differs in: over-all size of mandibular ramus, incisors, and molars smaller; anterior median fold of m1 present, though poorly developed.

From B. brachygnathus, B. rexroadi differs in: over-all size of mandibular ramus smaller; m3 larger; posterior cusps (hypoconid and entoconid) elongated; diastema shorter, less acutely recurved; incisors less prooedont; cingular ridges of m1 and m2 less well-developed.

From B. musculus, B. rexroadi differs in: over-all size of mandibular ramus less; cingular ridges of m1 and m2 less well-developed; incisors smaller, more prooedont; molars less depressed.

From B. taylori, B. rexroadi differs in: m3 more triangular, posterior part narrower; mental foramen closer to anterior root of m1; masseteric ridge closer to alveolus of m1; incisor shorter, more prooedont; molars more depressed.

Remarks.--Two maxillary tooth-rows and associated parts were studied. On one of these specimens, the M2 has a well-developed mesostyle; the anterior median fold of M1 is also well-developed. The other specimen possesses a low cingular ridge (enteroloph) between the protocone and the hypocone, a reduced cingular ridge (mesoloph) between the paracone and metacone of M1. On the second molar, M2, a mesostyle joins with the mesoloph somewhat in the fashion indicated by Hooper (1957:9, encircled number 2).

=Baiomys kolbi= Hibbard

Baiomys kolbi Hibbard, Trans. Kansas Acad. Sci., 55:201, June 18, 1952; Hibbard, Papers Mich. Acad. Sci., Arts and Letters, 38:403, April 27, 1953.

Type.--No. 24846, Univ. Michigan; right mandibular ramus bearing m1-m3 and incisor; Fox Canyon, upper Pliocene, Rexroad formation, Rexroad fauna, Univ. Michigan Locality K1-47, sec. 35, T. 34 S, R. 30 W, XI Ranch, Meade County, Kansas.

Referred material.--Univ. Michigan Nos. 24845-24848, 27494, 27497, 27499, 28566, 28861, 28878, 28880-28882, 28884, 28886.

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.

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.