EIGHTH ANNUAL REPORT. 
13 
and for the reception of which several new orders have been 
created. 
During the Lower Pliocene the mastodons entered South Amer- 
ica, and the Edentates, represented by the extinct ground sloths 
and the living armadillos, found their way into North America, 
together with a number of hystricomorph, or porcupine-like Ro- 
dents, which have since spread throughout the world. 
In the Upper Pliocene a more extensive interchange of animals 
took place, South America receiving from the North its camel- 
like llamas, its cats, its dogs, its raccoons, its numerous deer, and 
many others. The peccaries did not reach South America until 
the Pleistocene. 
The cats, represented by the sabre-toothed tigers, entering the 
southern continent at this time (Upper Pliocene) very possibly 
played a large part in the destruction of the giant herbivores, 
which flourished at that time on the Pampas. It would almost 
seem that the sabre-toothed tigers were modified in their mar- 
vellous dentition for the express purpose of preying on these huge 
and thick-skinned animals. 
South America, at this time, received its deer, all of which are 
closely related to Odocoileus, but which have been there long 
enough to evolve nearly twenty distinct species. 
The horses, too, entered South America during this period, 
and survived there nearly until the arrival of Europeans. 
Of the animals which North America received from the south- 
ern continent during the Pliocene, few have seemingly survived, 
except the porcupines and armadillos. The opossum did not 
come from South America, but is a survivor of a family of early 
Marsupials, Didelphidae, which were once widely spread through- 
out the northern hemisphere, but which have become extinct in 
the Old World. 
NORTH AMERICA. 
We have, in America, one family of the Carnivores, Procyoni- 
dae, and two distinct families of hoofed animals, Dicotylidae and 
Antilocapridae , and one very important genus of the deer family, 
Odocoileus , none of which have close relatives in the Old World. 
All these four groups are probably of autocthonous origin, and 
their peculiar characters are described at length below. 
EURASIA. 
The remaining large North American mammals, the wapiti, 
the bison, the musk-ox, the mountain sheep, the mountain goat, 
