244 2% SECTION — VERTÉBRÉS (SYSTÉMATIQUE) 
The Rodentia are of particular interest: they consist exclusively of 
Hystricomorpha; all the myomorphs, sciuromorphs and lagomorphs 
which now inhabit South America, came in the great migration from 
the North at the close of the Miocene. Of the six recent families of South 
American hystricomorphs, all but the Dinomyidæ and Dasyproctidæ are 
represented in the Santa-Cruz fauna and only one subfamily is now 
extinct. Several of the genera are plainly ancestral to living types, while 
others belong to extinct lines; it is interesting to observe that those ge- 
nera which were most flourishing, most abundant and most varied in 
Santa-Cruz times, are, for the most part, not those whose descendants 
persist to the recent epoch. 
Taken as whole, the Santa-Cruz rodent fauna is surprisingly modern, 
but in a number of anatomical details these genera are more primitive 
and serve to connect existing types with the ancient generalized forms 
of the Oligocene and Eocene. Some of these more generalized forms 
persist, with little modification, into the Miocene and throw much 
light upon the mutual relationships of the hystricomorphous families. 
Perhaps more interest attaches to the extraordinary series of Santa- 
Cruz ungulates than to any other group. These remarkable animals all 
belong to different orders from those which are found in the northern 
hemisphere and include neither Artiodactyla, Perissodactyla, Proboscidea, 
nor Amblypoda. Rorx' has lately published à very important paper, in 
which he shows that three of these ungulate groups agree in a very 
exceptional development of the auditory region, especially ofthe periotic, 
and that they should therefore be regarded as subdivisions of the same 
order, This conclusion is no doubt valid, though I must differ from 
Rorx’s opinions regarding the other two groups of Santa-Cruz ungulates, 
believing them all to be more nearly related to one another than he is 
disposed to admit. 
The following table represents my present ideas uponthe taxonomy of 
these animals : 
NOTOUNGULATA. 
I. Toxodontia. 
1. Toxodonta. 
2. Typotheria. 
3. Homaladotheria. 
Il. Astrapotheria. 
IT. Litopterna. 
The Toxodonta are extraordinarily abundant in the Santa-Cruz beds, 
where they are represented by larger and smaller species of the genus 
! Los Ungulados Sudamericanos. La Plata, 1904. 
