230 THE INTERNATIONAL MONTHLY. 
once inhabited all of the intervening region, but have become 
extinct everywhere, except in the two areas mentioned. This is 
an example of what is called " discontinuous distribution," and 
whenever the history of a group so distributed is known, it invari- 
ably proves to have been brought about just as in the case of the 
tapirs. 
A fourth perissodactyl family, which long ago utterly perished 
from the earth, is so remarkable that it should not be passed over 
in silence. This is the peculiarly North American family of the 
titanotheres. It was derived from the common perissodactyl 
stock and was distinctly separated from it as a recognizable fam- 
ily at least as early as the lower middle Eocene (Wind River 
beds). The oldest known representatives of it were rather small 
but stoutly built animals, with relatively large and heavy heads 
and peculiar upper molar teeth, which differ from those of most 
other perissodactyls in having no transverse crests, but conical 
tubercles on the grinding surface. In the succeeding Bridger 
beds, the members of this family are the commonest of all fossils and 
are represented by many species, of which great herds must have 
wandered on the shores of the lake. Only a few of these species, 
however, belong in the main line of descent, the others being 
collateral branches ; but they are all very much alike and differ 
only in minor details. The principal change from the Wind 
River forms is in the very notable increase in size and weight, 
although these Bridger animals (genus Telmatheriiun) are still far 
smaller than were their successors in the later geological stages, 
and resembled the modern tapirs in size and proportions. The 
canine teeth are sharp and formidable weapons, much resembling 
those of the bear ; the premolars are all simpler than the molars, 
and the lateral arches of the skull are strikingly prominent and 
thick. There are four digits in the front foot and three in the 
hind, as in almost all the perissodactyls of that time. It should 
be noted that in these animals, as in mammals generally, the long 
bones are hollow and contain cavities for the marrow. In species 
of the same genus from the upper Bridger and from the over- 
lying lower Uinta beds, indications of a pair of transversely placed 
