242 2° SECTION — VERTÉBRÉS (SYSTÉMATIQUE) 
cens age, as has, in my opinion been clearly demonstrated by the work of 
Dr. A. E. Orruanx upon the invertebrates of the underlying Patagonian 
formation. (See Vol. IV the Reports.) 
The Santa-Cruz formation itself is non-marine and largely of terres- 
trial origin, and where the fossil mammals are found, the matrix almost 
invariably consits of volcanic ash and tuff. This explains the surpri- 
singly large number of complete and nearly complete skeletons which 
are found; perhaps many of the animals were overwhelmed alive by the 
showers of ash and thus buried in an uninjured state. 
On looking over a series of representative Santa-Cruz mammals, we 
are immediately struck by the strangeness of the assemblage; not a 
single genus of these mammals occurs in any port of the northern hemi- 
sphere and, what is more surprising, the difference from the northern 
faunas is not only one of families and genera, but of orders. 
Thus the beds have yielded no Carnivora, Artiodactyla, Perissodactyla 
or Proboscidea, and no Rodentia except the Hystricomorpha. The place 
of the Carnivora was taken by the carnivorous marsupials, of which there 
is a great variety, more or less resembling the Australian Thylacynus. 
Numbers of dispropodont marsupials, mostly of extremely small size, 
accompany the carnivorous types, and of these Cænolestes is an interes- 
ting survival to modern times. 
One of the largest, most varied and most characteristic elements of the 
Santa-Cruz fauna is the subclass Edentata, which, so far as at present 
known, is represented only by three of its suborders, the Dasypoda, 
Glyptodontia and Gravigrada, Though no trace of the true sloths or of 
the anteaters has yet been found, it can hardly be doubted that both 
groups had already become differentiated, but they are to be sought for 
in some other region of South America. 
The armadillos are, for the most part, not ancestral to existing genera, 
but represent peculiar tvpes, some of which extended into the Pleisto- 
cene, while others died out in the Miocene. À peculiarity of the Santa- 
Cruz armadillos is the absence of a pectoral buckler, the carapace con- 
sisting chiefly of movable bands of plates, with a small posterior, or 
pelvic buckler, while in one genus (fide AmeGino) the entire carapace is 
made up of movable bands. 
The glyptodonts are relatively less advanced than the armadillos, and 
are strikingly smaller than their gigantic successors of the Pampean 
beds. The Santa-Cruz genera are not only smaller, but distinetly more 
primitive than the latter, as is shown in all parts of the exo- and endo- 
skeleton. The carapace always retains a greater or less number of par-- 
tially movable bands and the tail sheath is composed throughout of 
rings, never fused into a club-like mass. Several of the genera retain 
vestiges of incisors and the grinding teeth are mostly of a pattern sim- 
