THE EVOLUTION OF THE MAMMALIA. 241 
of things. There are a large number of genera of doubtful 
affinities and very varied in size, appearance, and structure, which 
abounded in the upper Eocene and Oligocene of Europe. A 
few of them migrated to North America in White River times, 
but the great majority of them have been found only in the east- 
ern hemisphere. Hardly anything is known of their origin and 
they probably were developed in some region not yet identified, 
and reached Europe by migration. Most of them had but a 
brief career, and died out, leaving no descendants. 
Another ungulate order, to which much interest attaches, is 
that of the Proboscidea, or elephants. Nothing whatever is known 
concerning the origin of these animals, but they cannot have 
taken their rise in either North America or Europe, for the group 
makes its appearance suddenly in the upper Miocene of these 
two continents, evidently as migrants from some other region^ 
which may have been Africa or southeastern Asia. At their 
first recorded appearance these curious and exceptional animals 
had already acquired the most distinctive and characteristic feat- 
ures of proboscidean structure, though in many details the Mio- 
cene forms are decidedly less advanced than the modern elephants. 
These forms have been referred to the genus Mastodon, but they 
should be separated as a distinct genus (TTetrabelodon), for they are 
quite different from the Pleistocene species to which the name 
Mastodon was first given. The Miocene species are decidedly 
smaller than the modern elephants, though they were large and 
massive creatures with body, limbs, and feet differing in no 
material respect from those of the elephants; the striking differ- 
ences are all in the dentition and the skull. The tusks are of 
moderate length and nearly straight, and instead of one pair only, 
there are two pairs, those of the lower jaw almost equaling the 
superior pair in length. The grinding teeth are all short crowned 
and of simple pattern, with three or four transverse ridges upon 
them and without a covering of cement. There is a complete 
milk dentition, and the permanent teeth succeed the temporary set 
vertically (that is, growing downward in the upper jaw, and upward 
in the lower jaw) as in normal mammals generally. The skull 
