. xi INTRODUCTION. 
in HKurope, Asia, or Africa. The types of Bradypus and 
Dasypus were, however, richly represented by diversified 
and gigantic specific forms in South America, during 
the geological period immediately preceding the present ; 
and fossil remains of extinct species of Cavia, Owlogenys, 
Ctenomys, and Cebus, have hitherto been detected exclu- 
sively in the continent where these genera still as ex- 
clusively exist. Auchenia more remotely typifies Macr- 
auchenia. The murine fossils in the rich collection of 
remains from Brazilian caverns, lately received at the 
British Museum, all belong to the genus Hesperomys, the 
aboriginal living representative of the Muride in South 
America; not a single fossil is referable to a true Old 
World Mus, though numbers of the common Rat and 
Mouse have been imported into South America since its 
discovery by Europeans. With regard to the Sloths and 
Armadillos, they now seem, after the rich harvest of 
bulky Glyptodons, Mylodons, Pachytheriums, and the 
more gigantic Megatherioid quadrupeds, to be the last rem- 
nants of a Mammalian Fauna, which once almost equalled 
in the size and number of its species that of the Europzo- 
Asiatic expanse, and was as peculiarly characteristic of 
the remote continent in which almost all its representa- 
tives have been entombed. 
In North America the most abundant Mammalian fos- 
sils of the corresponding recent geological epoch belong to 
a species of Mastodon (I. giganteus) peculiar to that con- 
tinent. Since, however, North America borders closely 
upon Asia at its northern basis, and is connected by its 
opposite apex with South America, it perfectly accords 
with the analogies of the geographical relations of the 
Jast-extirpated series of Mammals of the Old World that 
the Asiatic Mammoth and the South American Mega- 
