276 Universitjj of California FuMications in Zoology [Vol.21 



This synopsis is based on a stud}' of 698 specimens from 87 locali- 

 ties within the state of California. Besides this material, a topotype 

 series of Microtus mordax, as well as of other specimens from outside 

 the state, was also criticallj' examined. This group of meadow mice 

 in one or another of its geographic races occurs under suitable con- 

 ditions throughout the greater part of western North America, from 

 the San Bernardino Mountains of California and the Chusca Moun- 

 tains of New Mexico, north to Yakutat on the coast of Alaska and 

 to Fort Liard on the Liard Kiver in Canada, and east at least as far 

 as Custer, South Dakota. In most places within its range it is the 

 characteristic meadow mouse of the Canadian and Transition zones, 

 and in certain parts of its range it extends well up into the Arctic- 

 Alpine zone. 



ACKNOWLEDGMENTS 



The results contained herein are derived chiefly from a study of 

 the mammal collection of the Museum of Vertebrate Zoology. I am 

 especially indebted to Dr. Joseph Grinnell, director of the Museum, 

 under whose advice this problem was undertaken, and to Mr. Harry 

 S. Swarth, of the same institution, for aiding me in various ways. 

 Thanks are due, also, to Dr. Edward W. Nelson, Chief of the Bureau 

 of Biological Survey, United States Department of Agriculture, for 

 the loan of specimens, and to Mr. A. Brazier Howell for access to 

 important material in liis private collection. 



INDIVIDUAL VARIATION 



There are now known from the state of California more than 

 twenty-five species and subspecies within the genus Microtus, all 

 grouped under three subgenera, Lagnrvs, Chilotus, and Microtus. 

 In liabits the species are quite diverse, but they possess in common 

 certain structural characters which sharply distinguish them from 

 other living Microtinae. In many features of internal and external 

 structure, however, the genus Microtus is very similar to other genera 

 belonging to the subfamily. Certain modifications of the palate 

 usually serve to distinguish the genera from one another. 



With respect to the teeth, one finds on critical study that the char- 

 acters which ordinarily are of use in defining genera and species are 

 not so clear cut when applied to the Microtinae. Investigators in 

 recent years have shown that the elements of the enamel folding in 



