116 ZOOLOGICAL GEOGRAPHY. [part hi. 



remarkable and characteristic skunks is separated by Dr. J. E. 

 Gray as a genus — SpilogaW. In the American family Procyo- 

 nidse, a peculiar genus (Bassaris) is found in California and 

 Texas, extending south along the mountains of Mexico and 

 Guatemala. Eumetopias, and Halicyon, are seals confined to the 

 west coast of North America. The Bovidse, or hollow-horned 

 ruminants, contain three peculiar forms ; Antilocapra, the re- 

 markable prong-buck of the Eocky Mountains ; Aplocerus, a goat- 

 like antelope ; and Ovihos, the musk-sheep, confined to Arctic 

 America and Greenland. Among the Eodents are many pecu- 

 liar genera: Neotoma, Sigmodon, and Fiber, belong to the 

 Muridse, or rats ; Jaculus to the Dipodida?, or jerboas. The 

 very distinct family Saccomyidm, or pouched rats, which have 

 peculiar cheek pouches, or a kind of outer hairy mouth, con- 

 sists of five genera all confined to this region, with one of 

 doubtful affinities in Trinidad and Central America. In the 

 squirrel family (Sciuridse), Cynomys, the prairie-dogs, are pecu- 

 liar; and Tamias, the ground squirrel, is very characteristic, 

 though found also in North Asia. Haploodon, or sewellels, 

 consisting of two species, forms a distinct family ; and Ercthizon 

 is a peculiar form of tree porcupine (Cercolabidse) . True mice 

 and rats of the genus Mus are not indigenous to North America, 

 their place being supplied by a distinct genus (Hesperomys), 

 confined to the American continent. 



Birds. — The genera of birds absolutely peculiar to the Nearctic 

 region are not very numerous, because, there being no boundary 

 but one of climate between it and the Neotropical region, most 

 of its characteristic forms enter a short distance within the 

 limits we are obliged to concede to the latter. Owing also to 

 the severe winter-climate of a large part of the region (which 

 we know is a comparatively recent phenomenon), a large pro- 

 portion of its birds migrate southwards, to pass the winter in 

 the West-Indian islands or Mexico, some going as far as Guate- 

 mala, and a few even to Venezuela. 



In our chapter on extinct animals, we have shown, that there 

 is good reason for believing that the existing union of North 

 and South America is a quite recent occurrence ; and that the 



