2 + 2 THE INTERNATIONAL MONTHLY. 
is longer, lower, and more flattened than in the elephants, and has 
no such development of air sinuses around the brain case. 
In the lower Pliocene of India and the adjoining parts of 
southern Asia is found a transitional type, between the mastodons 
and the true elephants. In this intermediate type, the lower tusks 
have disappeared, and the grinding teeth have become much more 
complex ; the transverse ridges are much narrower and more 
numerous; the whole crown is covered with a deposit of cement, 
and grows longer before forming roots. In the true elephants 
(genus Elephas) which appear in the upper Pliocene, the grinding 
teeth become extraordinarily complex and grow much longer before 
the roots appear ; these teeth succeed one another from behind for- 
ward, the new tooth gradually pushing out the one in front of it ; 
an arrangement which is altogether exceptional among mammals. 
Only two of the grinders are in use on each side of each jaw, 
making eight in all, at one time, a method which supplies the 
elephant with fresh teeth through his very long life. 
In the Pleistocene the Proboscidea spread all over the world ; 
the mastodons seem to have died out in the eastern hemisphere 
at the end of the Pliocene, the true elephants taking their place ; 
but in North America the two genera flourished together and in 
South America only the mastodons occurred. As to so many 
other large animals, the events which followed the Glacial period 
were fatal to the Proboscidea over much the greater part of the 
earth's surface, and reduced them to their present limited range 
of Africa and southern Asia. A curious side branch of the Pro- 
boscidea is the genus Dinotherium, which was common in the 
Pliocene of Europe, but never reached North America. It had 
a long, low skull and no upper tusks, but only a lower pair which 
are bent downward and backward. 
Did space permit, many other series of hoofed animals would 
claim our attention and would offer much of interest and instruc- 
tion. But it is necessary to make a selection, and the lines 
described will suffice to illustrate the general character of ungu- 
late evolution. 
Turning now to the clawed mammals, we shall find the records 
