W.-B. SCOTT — MAMMALIAN FAUNA 245 
Nesodon; the number of these species has been much exaggerated, for, 
as AMeGuiNo has shown, the changes in the appearance of the animal, 
due to the developmental stages of the teeth, are most unusual. AME- 
GHINO has also pointed out the very remarkable fact that Nesodon has a 
complete and functional series of prelacteal incisors and canines. In- 
credible as this statement may appear, the evidence for it is very 
strong. 
Though perhaps not the actual and direct ancestor of the Pampean 
Toxodon, Nesodon yet shows us very nearly what that ancestor must 
have been. The skull is disproportionately heavy, the trunk massive, the 
limbs short and thick and the feet curiously small. The feet are tridactyl 
and, at first sight, very like those of the Perissodactyla, but the tarsus 1s 
of a radically different character and belongs to the type which all of 
the Santa-Cruz hoofed animals exemplify, without known exception. It 
should be added, however, that the fect of Astrapotherium have not yet 
been found. 
Even more abundant individually than the preceeding group and far 
more varied generically and specifically, is the suborder Typotheria. In 
the Santa-Cruz beds they are represented by a surprising number of 
small animals, with a great variety in dental and skeletal structure. The 
typotheres have a certain resemblance to the Hyracoidea, but this I be- 
lieve to be entirely analogical, and I quite agree with Rorx in the opinion 
that their closest relationships are with the toxodonts. 
The Homaladotheria are comparatively rare and their structure is 
much less completely known than it the case of the two preceeding su- 
borders, but sufficient has been learned to make clear their systematic 
position. The skull, dentition, carpus and tarsus, are fundamentally si- 
milar to those of the Toxodonta, yet the feet have undergone à curious 
modification, the position of the phalanges with reference to the meta- 
podials being quite unusual among the hoofed animals and the unguals 
having assumed the form of claws. À somewhatsimilar change has occur- 
red in the Ancylopoda and, among artiodactyls, in the Agriochæridæ, 
but the resemblance is merely another of those cases of convergence, 
which the Santa-Cruz fauna has in such a striking degree. 
The Astrapotheria still form an unsolved problem, for they are known 
only from the skull, no other part of the skeleton having been certainly 
identified. This is quite curious in view of the fact that skulls of Astra- 
potherium are not rare in Patagonia and in view of the large number of 
nearly complete skeletons of other animals which the collectors have 
obtained. 
Astrapotherium does not have the exceptional structure of the audi- 
tory region of the skull which characterizes the three preceding groups ; 
alone among the Santa-Cruz ungulates, it has large tusks in both jaws, 
