1914] Grinnell: Microdipodops 303 



is ashy gray, very faintly tinged with buff. This tone of coloration, 

 on the insides of the ears alone, serves to characterize every individual 

 in our series of polionotus. The white around the ears is particularly 

 conspicuous as compared with that in all the forms except califomicus, 

 in which species the darker surrounding color makes up for the lesser 

 amount of white in producing the effect of contrast. 



There is a notable difference in quantity and texture of pelage 

 among the different species. It is possible that some of the variation 

 in this respect is due to season, but it cannot all be. Our skins of 

 poliontus, taken in July, exhibit much longer pelage than is shown 

 by specimens of califomicus taken in August, and yet not so long and 

 especially not so silky and lax as shown by examples of pallidas taken 

 in May. 



The body size of all the species appears to be about the same, but 

 there is some variation in tail length. Polionotus has a lesser tail 

 length than pallidus as given by Merriam, resembling most nearly 

 megacephalus in this particular. 



The skulls of megacephalus and pallidus ai ssentially alike ac- 

 cording to Merriam. The skulls of polionotus differ from examples 

 at hand of those just named, in noticeably smaller auditory capsules. 

 The inflation is decidedly less, so that when looked at from behind the 

 skulls of polionotus do not stand so high, due to less inflation of bullae ; 

 when looked at from above the mastoid inflation is seen to be less pro- 

 tuberant both laterally and behind ; the notch between the bulging 

 capsules behind is thus not so deep. The rostra are not of the relatively 

 slender build shown in califomicus. 



Habitat. — The entire series of Microdipodops polionotus was ob- 

 tained on a sandy, sagebrush flat, on the McKeever Ranch, two miles 

 south of the railroad station of Benton, Mono County, California. The 

 altitude of this flat is close to 5200 feet, and in common with the rest 

 of the immediately adjacent region the type locality lies in an ex- 

 tremely arid faunal division of the Upper Sonoran Zone. The dates 

 of capture are July 10, 11, and 12. Three of the animals are obviously 

 young, one of these being not over half grown. 



Acknowledgments. — The present study was put upon a satisfactory 

 basis through the privilege accorded the writer by Mr. Henry W. 

 Henshaw, Chief of the Bureau of Biological Survey, Washington, of 

 examining specimens contained in the portion of the National collection 

 under his charge. This borrowed material consisted of skins-with- 

 skulls of each of the four previously described forms, as follows : 



