428 PR0CEEDIN08 OP THE NATIONAL MUSEUM. vol. 47. 



This revision of Onychomys is the result of the study of 1,562 

 specimens/ almost all of which have been modern well-made skins 

 with perfect skulls, A few alcoholic specimens and skeletons have 

 been examined. Except in a few cases the material has been suffi- 

 cient to work out the ranges of the subspecies in a fairly satisfactory 

 manner, though good series of specimens, including a sufficient num- 

 ber of adults, fi'om certam regions, will modify somewhat the bound- 

 ary lines between forms as at present mapped. All of the 23 existing 

 type-specimens have been examined. 



FORMS, CHARACTERS, AND PELAGES. 



One of the results of the reduction in the number of distinct species 

 to two, and the increase in the number of geographical races of each 

 of these, is the increased difficulty in finding hard and fast, easily 

 defined, conspicuous characters to diagnose these two species. 

 Wliile the two groups of subspecies as two specific units occupy an 

 immense range, one to the north and one to the south, the actual 

 area of overlapping is small.^ At these points of overlapping, where 

 intergradation could, but does not, take place, the two species are well 

 differentiated externally, there is never the least doubt concerning the 

 species to which a specimen belongs, and a working key to the species 

 in this overlapping area could be made on a great number of simple 

 differences, both external and cranial. Since all the numerous and 

 widely divei-sified forms have been found to intergrade indirectly 

 with either leucogaster or torridus, however, all of the superficial 

 characters which hold good to identify the species at the few over- 

 lapping points fail with some far removed form, which, although 

 intergrading indirectly with only one of tlie early named species, 

 may really resemble in any one or any combination of such char- 

 acters some race of the other species with which it has no real con- 

 specific relationship. The teeth present the best characters by 



> The material examined is from collections as follows: 



United States National Museum, Biological Survey collection 1, 041 



United States National Museum proper 163 



American Museum of Natural History, New York 118 



Museum of Vertebrate Zoology, University of California 83 



Field Museum of Natural History, Chicago 70 



Museum of Comparative Zoology, Cambridge 35 



Academy of Natural Sciences, Philadelphia 31 



University of Nebraska, Lincoln 15 



Kansas University Museum of Natural History, La\vrence 5 



Biological Survey of South Dakota 1 



Total number of specimens 1 , 562 



2 In only four cases do forms of the two species overlap in distribution, for comparatively limited areas 

 in each case, as follows: 



O. I. brevicaudus and 0. t. longkaudus in western Nevada and Mono County, California. 



O. }. ruidosx and 0. t. torridus in southwestern Arizona, northern Sonora, southern New Mexico, northern 

 Chihuahua, and extreme western Texas. 



O. I. arcticcps and 0. t. torridus in the Pecos Valley, New Mexico and Texas. 



O. I. albescens and 0. t. torridus in northern Chihuahua. 



