Part 3

United States National Museum, Division of Vertebrate Paleontology, C. Lewis Gazin.

University of Arizona, E. L. Cockrum, and G. VR. Bradshaw.

University of California, Museum of Vertebrate Zoology, Seth B. Benson, and W. Z. Lidicker.

University of Illinois, Museum of Natural History, Donald F. Hoffmeister.

University of Michigan, Museum of Zoology, W. H. Burt, E. T. Hooper, and Claude W. Hibbard.

University of New Mexico, James S. Findley.

University of Texas, Frank W. Blair.

Texas A & M, Cooperative Wildlife Research Collection, W. B. Davis.

The Museum, Michigan State University, Rollin H. Baker.

University of Florida Collections, James N. Layne.

I am especially grateful to Professor E. Raymond Hall who guided me in my study and gave critical assistance with the manuscript. Additional appreciated suggestions were made by Professors A. Byron Leonard, Robert W. Wilson, Henry S. Fitch, Ronald L. McGregor, and fellow graduate students. For the illustrations, I am indebted to Mrs. Lorna Cordonnier, Miss Lucy Remple and Mrs. Connie Spitz. Mr. B. J. Wilks of the University of Texas, Department of Zoology, provided a number of living pygmy mice for study in captivity. Mr. J. Raymond Alcorn and his son, Albert, collected a large share of specimens of pygmy mice now in the University of Kansas, Museum of Natural History. My wife, Patricia, aided me in secretarial work and typing of the manuscript.

For financial assistance, I am indebted to the National Science Foundation when I was a Research Assistant, to the Sigma Xi-RESA Research Fund for a Grant-in-Aid, and to the Kansas University Endowment Association through its A. Henley Aid Fund, and the Watkins Fund for out-of-state field work by the Museum of Natural History.

PALEONTOLOGY OF THE GENUS

Five fossil species, all extinct, have been assigned to the genus and range in time from early late Pliocene (Saw Rock Canyon fauna of Hibbard, 1953:408) to Mid-Pleistocene (see Hibbard, 1958:25, who assigns the Curtis Ranch fauna to late Kansan or early Yarmouth).

I examined all known fossil material and compared it with Recent material. When the antiquity of the genus is considered, the degree of difference between the oldest fossil species and the two living species is much less than might be expected.

=Baiomys sawrockensis= Hibbard

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

Baiomys rexroadi Hibbard, Amer. Midland Nat., 26:351, September, 1941; Hibbard, Contrib. Mus. Paleo., Univ. Michigan, 8(2):145, June 29, 1950 (part); Hibbard, Papers Mich. Acad. Sci., Arts and Letters, 38:403, April 27, 1953.