1897] THE SOURCE OF THE TERTIARY MAMMALIA 259 
kangaroos form part. A few years ago no one would have sus- 
pected that these latter could have taken their origin in any 
continent other than that of Australia, and still less in Argentina, 
separated to-day from the Australian lands by the immense abyss 
of the Pacific. 
These primitive Plagiaulacoidea or Diprotodonts were accom- 
panied by the Pyrotheria (Pyrotheriwm), mammals of very variable 
size, with pentadactyl feet, the limbs in the form of perpendicular 
columns of support, a short neck, large head, square grinding teeth 
with two transverse ridges like those of Dinotherium, large upper 
and lower tusks as in the oldest Mastodonts, and a large trunk like 
that of the elephant. They are the stock whence have sprung the 
proboscidians which appear completely developed on the Euro-asiatic 
continent in the Tertiary period, their origin until now having been 
an indecipherable enigma. 
Together with the Pyrotheria, there lived the Archaeohyracoidea 
(Archacohyrax, Argyrohyrax, etc.), small plantigrade mammals half- 
hoofed and half-clawed, whose external aspect was that of a cavy 
(Cavia), and which have given origin to the Hyracoidea (Hyrax) 
existing in Asia and Africa, whose ancestors have not been known 
until now in these continents. The Notohippidea (Morphippus, 
Thynchippus, eté.), small pentadactyl ungulates, but with the middle 
digit much larger than the side ones, constituted the stock from 
whence the horses have sprung. The Notostylopidea (Wotostylops, 
Trigonostylops, etc.), whose dentition has a rodent-like appearance, 
and give rise to the Tillodonts of the northern hemisphere. The 
Isotemnidea (Isotemnus, Trimerostephanos) which probably represent 
the source of all the ungulates. The Homalodontotheria (Asmodeus, 
ete.), the oldest ancestors of the extinct Ancylopoda of Europe, 
Asia, and North America, curious and anomalous herbivores which 
possessed all the characters of perfect ungulates, except in the 
digits, which were bent in the form of hooks and armed with com- 
pressed claws like the unguiculates. 
I have only mentioned a small portion of the ungulates of this 
period, which were very numerous. They were gigantic and with 
large tusks, like the Parastrapotheria, of medium size and generalised 
characters, like the Nesodonts and the Leontinidea; small, sturdy, 
and annectent forms between the ungulates and unguiculates, like 
the Hegetotheridea (Prohegetotherium), the Trachytheridea, and the 
Protypotheridea (Archaeophylus) ; tall and slender, like the deer, 
and with a single hoof on each foot imitating the horses in minia- 
ture, like the’ Proterotheridea (Deuterotheriwm), or with ambiguous 
affinities between the even and odd toed animals like Didolodus. 
Of these different groups some few have completely disappeared, 
and the rest have dispersed over the Argentine Territory, passed on 
