504 Prof. K. A. von Zittel—On the Mammalia. 
from older strata, in part new (those marked with *), but these 
represent, almost without exception, only advanced stages of differen- 
tiation of older types. But, in this fauna of the Southern world, one 
sees now, for the first time, a number of entirely foreign intruders 
intermingled, which betray a different descent. The genera Tapirus, 
Hippidium, Auchenia, Eoauchenia, Paraceros, Mastodon, and Canis 
have certainly not sprung from the soil of South America, but have 
migrated from the North, where they already existed, either as 
identically the same genera or as nearly related representative forms, 
in the Loup Fork beds. ‘ihis invasion of strangers from the North 
shows that the two halves of the Western continent first grew to- 
gether in the Pliocene period, and that probably at that time a 
land-bridge, broader even than the present Isthmus of Panama, joined 
together North and South America and the West Indies. 
But not only did the North American types make use of the 
newly opened way to widen the area of their distribution, but also 
the forms indigeuous to the South began to migrate to the North, 
and thus brought about one of the most remarkable instances of the 
overlapping of different faunas which geology has to record. In 
North America the so-called Equus-beds in the West and South-West 
of the United States (Oregon, California, Idaho, Arizona, New Mexico, 
Wyoming, Kansas, and Texas), in Mexico and Central America, and 
the contemporaneous Megalonyx-beds in the East (Kentucky, Penn- 
sylvania, Ohio, Carolina, Virginia, Florida), and in the West Indies 
(Cuba), represent either the last phase of the Tertiary period, or the 
beginning of the Pleistocene Diluvial period. A peculiar composite 
fauna of both Northern and Southern. descent peopled at that time 
North America, and left its remains in the deposits just mentioned. 
To the Northern types belong the genera Equus, Hipparion, Tapirus; 
Dicotyles, Platygonus; Auchenia, Eschatius, Holomeniscus, Cariacus, 
Cervus, Alces, Bos; Mastodon, Elephas; Castor, Erethizon, Castoroides, 
Sciurus, Arctomys, Jaculus, Arvicola, Thomomys, Geomys, Neotoma, 
Lagomys, Lepus, Scalops; Procyon, Arctodus, Putorius, Mustela, Lutra, 
Mephitis, Canis, Urocyon, Pachycyon, Chrysocyon, Arctotherium, Felis, 
Machairodus ; whilst to the South American types belong Megalonys, 
Mylodon, Glyptodon, Chlamydotherium, Hydrocherus, Amblyrhiza and 
Toxodon. 
Pampas Formation. 
The deposition of the Hquus- and Megalonyx-beds in North America 
may coincide with the formation of the loamy deposits, resembling 
the widely-distributed Loess, of the so-called “ Pampas Formation ” 
in Argentina and Uruguay. Also the volcanic tuffs of Bolivia, 
Peru and Chili contain mammalian remains which partly repeat 
those in the Equus-beds of Central America, and partly those in 
the Pampas mud. In its wealth of forms the fauna of the Pampas 
formation exceeds that now existing in Sonth America. It contains, 
according to Ameghino, 235 species and 98 genera. Hven if a part 
of the species indicated in his lists will not stand a more critical 
review, there will yet remain so large a residue of good genera and 
