Baiomys sawrockensis Hibbard, Papers Mich. Acad. Sci., Arts and Letters, 38:402, April 27, 1953.
Type.—No. 27506, Univ. Michigan; left mandibular ramus bearing m1-m3 and incisor; Saw Rock Canyon, early late Pliocene, XI member of the Rexroad formation, sec. 36, T. 34 S, R. 31 W, Seward County, Kansas (University of Kansas, Locality 6).
Referred material.—Univ. Michigan, Nos. 25781, 27503-27505, 28159-28165, 29708-29715, 31015.
Diagnosis.—Ramus of medium size to small for the genus; lower incisor broad, moderately recurved; diastemal region broad; anterior median fold between anterior labial conulid and anterior lingual conulid of m1 deep; primary first fold between anteroconulid and protoconid of m2 deep; cingular ridge (ectolophid) at entrance to posteroexternal reëntrant valley (major fold, see [Figure 2]) between protoconid and hypoconid of m1 and m2; average and extreme measurements of lower molar row of eight specimens are, 2.65 (2.5-2.7).
Comparisons.—For comparisons with B. brachygnathus, see account of that species. From B. rexroadi, B. sawrockensis differs in: anterior median fold of m1 deeper; incisor narrower; diastemal region broader; coronoid process broader and better developed; cingular ridges (ectolophids and mesolophids) more pronounced in their development; incisors less proödont, more retrodont.
From B. kolbi, B. sawrockensis differs in: crowns of molars narrower; incisors less proödont; cingular ridges (ectolophids and mesolophids) of m1 and m2 more pronounced in their development.
From B. minimus, B. sawrockensis differs in: incisor less procumbent; masseteric ridge extending farther anteriorly; anterior cingulum of m2 slightly larger.
From B. musculus, B. sawrockensis differs in: over-all size of jaw and molar row less; diastema more acutely curved; incisors shorter; anterior median fold of m1 slightly deeper.
From B. taylori, B. sawrockensis differs in: m1 and m2 smaller; cingular ridges in m1 and m2 more pronounced; anterolingual conulid farther forward; incisors shorter, more proödont; molar teeth depressed, less hypsodont; diastemal region broader, more acutely curved; masseteric ridge not extending so far anteriorly.
Remarks.—B. sawrockensis is the oldest known pygmy mouse. The extreme development of the anterior median fold between the anterolingual conulid and the anterolabial conulid is regarded as a primitive feature in the pygmy mice. In this character, the Recent species can be traced back in time through B. minimus to B. sawrockensis. B. sawrockensis resembles Calomys laucha of South America in general conformation of jaw and tooth structure. The molars of sawrockensis are smaller than those of C. laucha, and the anterolingual conulid of sawrockensis is farther forward.