23* THE INTERNATIONAL MONTHLY. 
much like those of the elephants ; as in the latter, the long bones 
have lost their marrow cavities, which are replaced by a tissue of 
spongy bone, a feature usual in very large animals. The feet 
have lost no digits, and are not so much shortened as in the ele- 
phants, but the hoofs are reduced to just such nodules as in 
them. 
Altogether, the titanotheres are a very curious and interesting 
family, and it is not easy to see why none of its members should 
have reached the Old World, when other perissodactyls migrated 
backward and forward between the continents so freely. 
A second great order of hoofed animals, and one which is now, 
and long has been, dominant is that of the artiodactyls, or even- 
toed ungulates, which include such familiar forms as the pigs, 
hippopotamus, camels, deer, antelopes, sheep, oxen, etc. If we 
should gather together all of the living and extinct forms of artio- 
dactyls, we should have a vast assemblage of families and genera, far 
exceeding in numbers and variety those of the perissodactyls, and 
it is, therefore, impossible to consider more than a very few of 
the more important groups here. Indeed, the mutual relation- 
ships of this great and heterogeneous assemblage of animals con- 
stitute some of the most complex and difficult problems of zool- 
ogy (" the despair of the zoologist," they have been called), and 
they cannot be determined until the history of each group has 
been traced step by step through a reasonably complete series of 
fossil forms. 
The order makes its first recorded appearance in the Wasatch 
of North America, though our ignorance of the contemporaneous 
fauna of Europe forbids the inference that they had not already 
reached that continent also. In the upper Eocene of Europe, 
the artiodactyls are already so numerous, so diversified, and so 
different from those of America, that they must be regarded as 
migrants from some region not yet determined. In all probabil- 
ity, the artiodactyls were derived from the Condylarthra, the same 
group as that which gave rise to the perissodactyls, though the 
connection cannot yet be demonstrated in an altogether satisfac- 
tory way. 
