ON THE EXTINCT ANIMALS OF NOHTH AMERICA. 
277 
Philadelphia, and Professor 0. C. Marsh of Yale College, have 
taken the subject in hand, both as explorers and describers.* 
It must be premised that the material has come to hand so 
rapidly during the last three or four years that most of the 
information which has hitherto been given to the world, espe- 
cially by the two last-named palaeontologists, is in a very pro- 
visional and fragmentary state ; and that until the flood of new 
discovery begins to ebb, and the few labourers in this plentiful 
harvest-field have leisure to prepare careful, elaborate, and, 
above all, well-illustrated descriptions of the specimens, we 
shall remain in much uncertainty about the real nature and 
relations of many of the animals of that strange old fauna, 
which at present are to us little more than names. 
I must presume that readers are familiar with the main 
epochs of time into which geologists have divided the earth’s 
history. For the present purpose we shall only have to refer 
to the latest of these, the Tertiary, representing how many 
millions of years we cannot say ; and which, for convenience, is 
generally subdivided into four sub-epochs, the Eocene, the 
Miocene, the Pliocene, and the Pleistocene, the end of which 
brings us to the period in which we now dwell. Of course it is 
not implied by this division that there was any sudden break 
or interruption of the steady course of the world’s progress 
between these periods. They are merely artificial and arbi- 
trary, though convenient stages, and pass insensibly one into 
the other ; but without the use of some such terms we could 
not fix the epoch of any particular event or set of events. In 
geology we know nothing of centuries. We have no kings’ 
reigns, as in political history, to mark the course of time, so 
we speak of 44 Miocene ” much in the same vague kind of sense 
in which we speak of the 44 Middle Ages ” in our chronology of 
the historical events in Europe. 
The first evidence of mammalian remains in strata of Miocene 
age in Western America was that made known in 1846 by Dr. 
Hiram A. Prout, of teeth then supposed to belong to a gigantic 
species of Pcclceotherium , and subsequently described by Leidy 
under the name of Titanotherium. This was the commence- 
ment of that interesting series of discoveries, which have now 
made the 64 Mauvaises Terres,” or 44 Bad Lands,” of the White 
Eiver of Dacota, classical ground to the palaeontologist. But 
it was not until 1869 that the older beds on the western side of 
the Rocky Mountains were explored, and the more ancient 
Eocene land fauna of North America brought to light. In that 
* I am glad to take the opportunity of thanking Professors Hayden, 
Leidy, Marsh, and Cope, for their kindness in sending me copies of all their 
numerous memoirs hearing upon the subject of this article. 
