356 GEOGRAPHICAL AKD GEOLOGICAL DISTRIBUTION. 



continent. The wood-rats, constituting the genus Neotoma, are 

 the largest of the American murine forms, and inhabit the greater 

 portion of the region included between Guatemala and Canada. 

 The cotton-rat (Sigmodon hispidus) is confined principally to the 

 Southern United States, Mexico, and Central America, occasionally, 

 it appears, wandering into the northern portions of South America. 

 In the genus Reithrodon are included a limited number of remarka- 

 ble leporine forms, which, though differing very essentially in gen- 

 eral appearance, do not seem to be distantly removed from the 

 North American harvest-mice of the genus Ochetodon; they are 

 confined principally to the extremity of the South American con- 

 tinent and to Tierra del Fuego. Interesting modifications of struct- 

 ure or habit are seen in some of the murine forms, as in the par- 

 tially web-footed water-rats of the Australian region (Hydromys), 

 and in their Brazilian analogues of the genus Holocheilus ; again, 

 in the arboreal dormouse-like forms that have been referred to the 

 genera Dendromys (Ethiopian) and Rhipidomys (American). 



Of the less murine or rat-like forms of the mouse family the 

 voles or meadow-mice (Arvicolae), which in a measure replace the 

 true mice in the far north, and on elevated mountain-summits, and 

 whose distribution embraces nearly the whole of temperate and 

 Arctic Eurasia and North America, are probably the most nu- 

 merous specifically, while in point of individual numbers they far 

 exceed any other mammal, with the possible exception of the 

 closely related lemming. Many of the species enjoy a very broad 

 distribution, but none are known to be common to both the Eastern 

 and Western Hemispheres. Arvicola arvalis or agrestis, the com- 

 mon meadow-mouse, which ascends the Alps to a height of 7,000 

 feet, is distributed over nearly the whole of Europe (including 

 Italy) and Siberia, its range corresponding approximately with that 

 of A. amphibia (water-vole) ; A. alpina or nivalis inhabits the re- 

 gion of the higher Alps, between 5,000 and 12,000 feet elevation; 

 on the Finster-Aarhorn it lias been observed at an altitude of 4,000 

 metres. The most broadly distributed of the American species is 

 the common meadow -mouse (A. riparia), whose range extends from 

 the Atlantic to the Pacific, and from the Carolinas to the Hudson 

 Bay territory. A single species, A. quasiater, is known from Mex- 

 ico. Evotomys rutilus, a form very closely related to the arvicoles, 

 inhabits the circumpolar regions of both hemispheres. 



