MAMMALS OF tfORTH AMERICA. 63 



eludes the common or domestic rats and mice, and which is rep- 

 resented on all the grand divisions of the Eastern Hemisphere, is 

 completely wanting, not only in North America, but in the entire 

 New World, where its place is taken by the closely-allied vesper- 

 mice, constituting the genus Hesperomys. The musk-rat (Fiber), 

 belonging to the same family, is not found outside the limits of the 

 North American continent, although its range extends into the 

 Neotropical realm (Mexico). The squirrels (Sciuridse) are princi- 

 pally Old World forms ; they comprise the true squirrels (Sciurus), 

 flat-tailed flying squirrels (Sciuropterus), ground-shrews (Tamias), 

 marmots (Arctomys), and pouched-marmots, or spermophiles (Sper- 

 mophilus). In addition to these forms we are presented with the 

 curious animal known as prairie-dog (Cynomys), whose range is 

 confined to the central continental region. Among other rodents 

 may be mentioned the jumping-rat (Jaculus, or Zapus), allied to 

 the eastern jerboas, and the Canadian porcupine (Erethizon), be- 

 longing to a group of animals (Cercolabidae) distinguished from 

 the true or Old World porcupines both structurally and in their 

 arboreal habits. The ungulates, or hoofed animals, have but a very 

 feeble development in the Nearctic division of the Holarctic realm. 

 The goats and sheep are, with two exceptions, the big-horn (Ovis 

 montana), an inhabitant of the Rocky Mountains, and the musk-ox 

 (Ovibos moschatus), from the Arctic district, completely wanting, 

 a faunal characteristic which eminently serves to distinguish the 

 western division of the Holarctic tract from the eastern, to which 

 almost the whole of this group of animals is confined. The ante- 

 lopes are limited to two species, representing two distinct types, 

 both of them confined to the more temperate regions of the conti- 

 nent. The one is the "prong-horn" of the Western plains (Anti- 

 locapra), and the other the Rocky Mountain goat (Aplocerus lani- 

 ger), which, as the name indicates, is partial to the mountain 

 fastnesses. Two varieties of the bison, or American buffalo, are 

 recognised the buffalo of the plains, and the buffalo of the for- 

 ests and mountains ; but the variation observable between these is 

 one pertaining to habit and not to structure, and therefore not of 

 specific importance. The Carnivora present several distinctively 

 American types, and notably so the raccoons (Procyonidae), a small 

 group of interesting quadrupeds, whose home is primarily the 

 region of the tropics, and which appear to hold a somewhat inter- 



