46 PATAGONIAN EXPEDITIONS: ZOOLOGY. 



of interest soon to secure large series of specimens from widely separated 

 localities where it has been long established for the purpose of determin- 

 ing what modifications it may have undergone through the influence of 

 very diverse conditions of environment. That strongly-marked differen- 

 tiation will be obvious is evident from what is already known to have 

 taken place among these mice inhabiting the fields in the vicinity of 

 Jalapa, Mexico, where a well-marked black phase [Mus nmscidus jalapce 

 All. & Chapm.) has already been developed.^ 



Genus ORYZOMYS Baird. 



Orysomys Baird, Mamm. N. Am., 1858, 458 (as a subgenus of Hesper- 



omys). Type and only species, Miis pahtstris Harlan. 

 Zygodontomys Allen, Bull. Am. Mus. Nat. Hist, IX, 38, pi. i, figs. 1-7, 

 March 11, 1897. Type, Oryzomys cherriei Allen. Described as a 

 genus, but more commonly treated as a subgenus of Oryzomys. 

 Oligoryzomys Bangs, Proc. N. Engl. Zool. Club, I, 94, Feb. 23, 1900 (as 



a subgenus of Oryzomys). Type, Oryzomys naviis Bangs. 

 Erioryzomys Bangs, Proc. N. Engl. Zool. Club, I, 96, Feb. 23, 1900 (as 



a subgenus of Oryzomys). Type, Oryzomys juonochromos Bangs. 

 Melanomys Thomas, Novitates Zoolog., X, 41 (in text), April, 1903 (as a 

 subgenus of Ory.S'C'wj/^). Type, Oryzotnys p/ia^opus ThoxndiS. 

 The genus Oryzomys, as commonly recognized, is the most abundant 

 and most widely distributed genus of American Muridas, it ranging from 

 the warmer parts of the southeastern United States to Tierra del Fuego. 

 Altogether about 185 species and subspecies have been referred to it, and 

 the list is still rapidly increasing by the discovery and description of addi- 

 tional forms. Although the group is far from homogenous, the transitions 

 from one type to another are usually by gradual stages. In size the 

 various species range from the size of a house mouse to that of the large 

 brown rat ; the tail may be much more than half the total length of the ani- 

 mal or less than one quarter ; the supraorbital ridges may be heavily devel- 

 oped or obsolete ; and the enamel pattern of the teeth is quite variable. 

 As shown in the synonymy above, several minor groups have been set 

 off as subgenera, with fairly well marked characters, and, owing to the num- 

 erical unwieldiness of the genus, it will probably be found convenient, 

 sooner or later, to employ them in the sense of full genera. 

 • Cf. Bull. Am. Mus. Nat. Hist, IX, 1897, p. 198. 



