_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 reentrant 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 prooedont, more retrodont. From _B. kolbi_, _B. sawrockensis_ differs in: crowns of molars narrower; incisors less prooedont; 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 prooedont; 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. =Baiomys rexroadi= Hibbard