align="left">SDM.

San Diego Natural History Museum. SITC. Southern Illinois Teachers College. USBS. United States Biological Surveys Collection. USNM. United States National Museum. UCM. University of Colorado Museum. UIM. University of Illinois Museum of Natural History. UM. University of Michigan Museum of Zoology. UU. University of Utah Museum of Zoology.

The species are arranged from least to most progressive, and the subspecies are arranged alphabetically.

The synonymy for each subspecies includes first a citation to the earliest available name then one citation to each name combination that has been applied to the subspecies and, finally, any other especially important references.

Marginal records of occurrence for each subspecies are shown on the maps by means of hollow circles and these localities are listed in clockwise order beginning with the northernmost locality. If more than one of these localities lies on the line of latitude that is northernmost for a given subspecies the western-most of these is recorded first. Marginal localities have been cited in a separate paragraph at the end of the section on specimens examined in the account of a subspecies. Localities that are not marginal are shown on the maps by solid black circles. Localities that could not be represented on the distribution map because of undue crowding or overlapping of symbols are italicized in the lists of specimens examined and in the lists of marginal records.

The localities of capture of specimens examined are recorded alphabetically by state or province, and then by county in each state or province. Within a county the specimens are recorded geographically from north to south. The word “County” is written out in full when the name of the county is written on the label of each specimen listed for that county, but the abbreviation “Co.” is used when one specimen or more here assigned to a given county lacks the name of the county on the label.

The following account has been made possible only by the kindness and cooperation of those persons in charge of the collections listed above. For the privilege of using the specimens in their care I am deeply grateful, as I am also to Prof. A. Byron Leonard for assistance with figures 35-37, to Dr. Rufus Thompson for figures 16-21, and to Mr. Victor Hogg who made all of the other illustrations. My wife, Dorothy Krutzsch, helped untiringly in assembling data, in typing the manuscript, and gave me continued encouragement. Finally, I am grateful to Professor E. Raymond Hall for guidance in the study and critical assistance in the preparation of the manuscript and to Professors Rollin H. Baker, Robert W. Wilson, and Robert E. Beer for valued suggestions.

The fossil record of the genus Zapus is scanty. All of the known fossils of it are lower jaws of Pleistocene Age. The Recent species Z. hudsonius was recorded by Cope (1871:86) in the Port Kennedy Cave fauna (pre-Wisconsinian) of Pennsylvania. Gidley and Gazin (1938:67) reported a single mandibular ramus bearing m1-m3 recovered from the Cumberland Cave (pre-Wisconsinian) of Maryland. The teeth are not typical of modern Zapus in that m1 and m2 are shorter crowned and m1 has a longer anterior lobe. Gidley and Gazin, nevertheless, considered their material insufficient for establishing a new species.

Two extinct species have been described: Zapus burti Hibbard (1941:215) from the Crooked Creek formation (= Meade formation of the State Geological Survey of Kansas) mid-Pleistocene of Kansas and Zapus rinkeri Hibbard (1951:351) from the Rexroad formation (= Blanco formation of the State Geological Survey of Kansas) of Blancan Age of Kansas. Both species resemble Zapus hudsonius, but differ from it in broader crowned more brachydont cheek-teeth. Z. rinkeri differs from Z. burti and Z. hudsonius by a more robust ramus, broader molars, and three instead of two internal re-entrant valleys posterior to the anterior loop on m1. The three species Z. rinkeri, Z. burti, and Z. hudsonius are in a structurally, as well as a geologically, progressive series. The trend in dentition is from broad, brachydont cheek-teeth to narrow, semi-hypsodont cheek-teeth.

The subfamily Zapodinae is known from Pliocene and Pleistocene deposits of North America and now occurs over much of northern North America and in Szechuan and Kansu, China. The living species occur among grasses and low herbs in damp or marshy places both in forested areas and in plains areas.

The early Pliocene Macrognathomys nanus Hall (1930:305), originally described as a Cricetid, is actually a Zapodid as shown by the structure of the mandibular ramus, shape of the incisors, and occlusal pattern of the cheek-teeth.

If Macrognathomys can be considered a member of the subfamily Zapodinae (possibly it is a sicistine) then it represents the oldest known member of this subfamily. Judging from the published illustrations, Macrognathomys seems to be structurally ancestral to the Mid Pliocene Pliozapus solus Wilson; the labial re-entrant folds are wider and shorter and on m2 and m3 fewer. The difference in stage of wear of the teeth in Macrognathomys and Pliozapus is a handicap in comparing the two genera but they are distinct. Wilson (1936:32) points out that Pliozapus clearly falls in the Zapodinae and stands in an ancestral position with respect to the structurally progressive series Eozapus, Zapus, and Napaeozapus. Nevertheless, Pliozapus cannot be considered as directly ancestral to Eozapus because of the progressive features in the dentition of Pliozapus. Wilson (1937:52) remarked that if Pliozapus is ancestral to Zapus and Napaeozapus, considerable evolution must have taken place in the height of crown and in the development of the complexity of the tooth pattern. In contrast to Wilson’s opinion, Stehlin and Schaub (1951:313) placed Pliozapus and Eozapus in the subfamily Sicistinae because certain elements in the occlusal pattern of the cheek-teeth are similar. I disagree with those authors and hold with Wilson; I consider Pliozapus and Eozapus in the subfamily Zapodinae. In dental pattern Pliozapus, as Wilson (1936:32) pointed out, resembles the Recent Eurasiatic sicistid, Sicista more than do Zapus or Napaeozapus. Nevertheless, from Sicista Wilson distinguishes Pliozapus and relates it to the subfamily Zapodinae by: "more oblique direction of protoconid-hypoconid ridge, anterior termination of this ridge at buccal portion of protoconid rather than between protoconid and metaconid as in Sicista; cusps more compressed into lophs; cheek-teeth somewhat broader; greater development of