116 
ZOOLOGICAL GEOGRAPHY. 
[PART III. 
remarkable and characteristic skunks is separated by Dr. J. E. 
Gray as a genus — Spilogale. In the American family Procyo- 
nidas, a peculiar genus (Bassaris) is found in California and 
Texas, extending south along the mountains of Mexico and 
Guatemala. Fumetopias, and Halicyon , are seals confined to the 
west coast of North America. The Bovidae, or hollow-horned 
ruminants, contain three peculiar forms ; Antilocapra , the re- 
markable prong-buck of the Bocky Mountains ; Aplocerus , a goat- 
like antelope; and Ovibos, the musk-sheep, confined to Arctic 
America and Greenland. Among the Bodents are many pecu- 
liar genera: Neotoma , Sigmodon, and Fiber , belong to the 
Muridae, or rats; Jaculus to the Dipodid®, or jerboas. The 
very distinct family Saccomyidce , 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 (Sciuridae), 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 Frethizon 
is a peculiar form of tree porcupine (Cercolabkke) . True mice 
and rats of the genus Mus are not indigenous to North America, 
their place being supplied by a distinct genus {Hespei'omys ) , 
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 
