SCOTT: TOXODONTIA OF THE SANTA CRUZ BEDS. 297 
The Systematic Position of the Toxodontia. 
This difficult problem can be dealt with here only in a brief and tenta- 
tive manner, reserving the full discussion to a subsequent volume, after 
the description of the Astrapotheria shall have appeared, for the problem 
of the Toxodontia is that of all the indigenous groups of South American 
ungulates. 
It is not surprising that different observers should have reached very 
divergent conclusions, for the difficulty of giving the true valuation to the 
long lists of likenesses and unlikenesses among the various groups is 
exceedingly great. Indeed, a definitive solution of the problem is hardly 
to be hoped for until much more is learned regarding the skeletal struc- 
ture of the pre-Santa Cruzian genera, especially those of the Notostylops 
Beds. 
In his later writings, Ameghino attempted to bring nearly all the pecu- 
liarly South American groups of hoofed animals into relation with the 
orders of the northern hemisphere ; the Litopterna were united with the 
Perissodactyla, the Astrapotheria with the Amblypoda, the Pyrotheria with 
the Proboscidea and the Entelonychia with the Ancylopoda, except the 
Notostylopidae, which are referred to the Tillodonta. The Notohippidae 
are removed from the toxodonts and united with the horses to form a new 
order, the “ Hippoidea.” The Toxodontia are retained as a distinct order 
and no very definite relationship is suggested. An almost diamet- 
rically opposite view is that of Lydekker (’96) who considers that all the 
South American ungulates, except perhaps the Pyrotheria, are more or 
less closely related and derived from a common stock. In particular, the 
astrapotheres and homalodontotheres are regarded as nearly related and 
are included in the same order. Roth, in the remarkable paper so often 
quoted (’03), refers the Litopterna to the perissodactyls and apparently, 
though not explicitly, is inclined to make the same reference of the Astra- 
potheria. The toxodonts, in the narrow sense, the typotheres and the 
homalodontotheres are included in a new order, the Notoungulata, which 
Roth believes to be widely separated from all other hoofed animals and 
related to the Primates. Gregory (To) extends Roth’s term to include all 
the indigenous South American forms, even the Pyrotheria, and raises the 
Notoungulata to a super-order. Schlosser’s (Ti) classification of the 
hoofed mammals is an unusual one, admitting only three orders and rele- 
gating the other groups to subordinal rank. The three orders are I Ungu- 
