102 Bailey North American Species of Sigmodon. 



about 1000 specimens in the Biological Survey collection and 

 the private collection of Dr. C. Hart Merriam, both in the 

 United States National Museum, besides a number of types and 

 topotypes from other sources, for the use of which I am in 

 debted to Dr. F. W. True, executive curator, and Mr. Gerrit 

 S. Miller, Jr., assistant curator of mammals, in the U. S. 

 National Museum; to Dr. J. A. Allen, curator of mammals and 

 birds, American Museum Natural History; and to Mr. Outram 

 Bangs, curator of mammals, Museum of Comparative Zoology. 



The North American species of Sigmodon fall naturally into 

 two groups, which may be designated the hispidus group, and 

 the hairy- tailed group. The former, of which hispidus was the 

 first named species, is characterized by a semi-naked tail, the 

 coarse annulations of the epidermis being scarcely obscured by 

 the short scattered hairs. The forms of this group inhabit 

 the Lower Sonoran and Tropical zones, are widely distributed, 

 with an almost continuous range, and, with the one exception 

 of an overlapping species (alleni) are here treated as subspecies 

 of hispidus. In general the pelage is coarse and hispid, usually 

 much more so in summer than in winter, but this character 

 varies considerably with the various forms. The group includes 

 hispidus, littoralis, spadicipygus, texianus, berlandieri, eremi- 

 cus, arizonce, mascotensis, tonalensis, major, toltecus, satura- 

 tus, microdon, borucce, and alleni. 



The hairy-tailed group, of which fulviventer was the first 

 known species, is characterized by a moderately hairy tail, the 

 short, bristly hairs almost or quite concealing the underlying 

 annulations. In most of the species the pelage is softer than 

 is usual in the hispidus group. The group is not distinguished 

 by cranial characters and the species differ as widely among 

 themselves as they do from those of the hispidus group. So 

 far as ascertained they inhabit the Upper Sonoran and Transi 

 tion zones in widely scattered mountains in Mexico and adjoin 

 ing parts of the United States. In most cases they appear to 

 be isolated by surrounding areas belonging to lower zones. 

 Most of the species seem to have a very limited distribution 

 and are distinguished by strongly marked characters. The 

 group includes fulviventer ', melanotis, minimus, ochrognathus, 

 leucotis, alticola and amoles, all but the last of which are 

 treated as full species. 



