ON THE EXTINCT ANIMALS OF NOETH AMEEICA. 
285 
(with many other Old World forms, however) completely break 
down the line of demarcation which our knowledge, when 
limited only to the existing fauna of the world, caused zoolo- 
gists to draw between those two groups. 
They appear to have survived throughout the whole of the 
Miocene period, commencing in the genus called Agriochcerus 
in the uppermost Eocene, and ending in the Merychyus of the 
early Pliocene ; and it is of great interest to know that a 
gradual modification can he traced in the characters of the 
animals of the group, corresponding with their chronological 
position, from the earlier more generalised to the latest com- 
paratively specialised forms, thus affording one of the most 
complete pieces of evidence that is known in favour of a pro- 
gressive alteration of form, not only specific, hut even of 
generic importance, through advancing ages. 
Another group of great interest made its appearance in the 
Miocene of North America, and which, if the evidence of frag- 
ments can be trusted, did not become extinct, like the last, but, 
continuing to exist through the Pliocene and Pleistocene ages, 
is still represented on two distant parts of the earth by the 
three or four species of llama of South America, and the two 
species of camel of the Old World. The discovery of the 
early Miocene Poebrotherium and of the Pliocene Procamelus 
and Pliauchenia — remains of which, and of Pleistocene Auche - 
nice of great size, though not generically distinguishable from 
the modem llamas, are abundantly distributed over the North 
American continent — seem to show that that country was the- 
original home of the singular family of Gamelidce, which was 
probably introduced by emigration in its perfect condition into 
the Old World, where none of the transitional forms from the 
more generalised ruminants, like those above mentioned, have 
been met with. 
On the other hand, of the gigantic four-horned Sivatherium 
of the Himalayas, the equally large but hornless Helladotherium 
of the Grecian Miocene, or of the giraffes, no representatives 
have hitherto been found in America. And very little light has 
been shed upon the origin and distribution of the true rumi- 
nants by these researches, except in a negative manner. No- 
deer have been found in the Miocene (at which epoch they 
were abundant in Europe) ; in the Pliocene but a single and 
poorly developed species ; while in the Pleistocene, with tho 
exception of one large species, called Gervus Americanus , they 
scarcely differ from those of the actual fauna. Of the hollow- 
horned ruminants, several species of bison, of Ovibos or musk 
ox, and a single Ovis , have been described, all from the Pleis- 
tocene, but not a single species of antelope. From these facts 
it may safely be inferred that the few existing and Pleistocene- 
