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 {Pyrothcrium), 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 Dinotheriitm, 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 

 {Archa colli/ rax, Argyrohyrax, etc.), small plantigrade mammals half- 

 hoofed and half-clawed, whose external aspect was that of a cavy 

 (Cavi«), 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, 

 Rhynchippus, etc.), 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 (Notostylops, 

 Trig<mostylops, etc.), 'whose dentition has a rodent-like ■ appearance, 

 and give rise to the Tillodonts of the northern hemisphere. The 

 Isotemnidea {Isotemnus, Trimcrostcphcmos) which probably represent 

 the source of all the ungulates. The Homalodontotheria (Asmodeus, 

 etc.), 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 (Prohegctothei-iiim), the Trachytheridea, and the 

 Protypotheridea (Arcliaeophylus) ; tall and slender, like the deer, 

 and with a single hoof on each foot imitating the horses in minia- 

 ture, like the Proterotheridea (Deuterotherium), or with ambiguous 

 affinities between the even and odd toed animals like Didolodiis. 



Of these different groups some few have completely disappeared, 

 and the rest have dispersed over the Argentine Territory, passed on 



