1897] THE SOURCE OF THE TERTIARY MAMMALIA 263 
summit of their development, to end in those gigantic beings whose 
skeletons fill the galleries of the Museums of Buenos Aires and La 
Plata. The two Americas had been separated until now by the ocean, 
and the territories of Panama and Central America had been sub- 
merged in a deep sea which put the Atlantic and Pacific in com- 
munication. 
Great tectonic movements produced a general raising of the 
mountain chains which traverse the New World from south to 
north, followed by a great retreat of the waters of the ocean. 
The continental mass acquired a larger extension, and both Americas 
were put into communication by the raising of a vast land-surface, 
in which to-day are the Gulf of Panama and the Caribbean Sea. 
The Galapagos Islands on one side and the Antilles on the other 
remained surrounded in this newly-risen land, and America in the 
form of a great rectangular continental mass extended from pole to 
pole. ; 
The terrestrial faunas, confined hitherto by the inter-American 
sea, on the disappearance of this barrier began to move in opposite 
directions, that of the north towards the south and that of the south 
towards the north, producing a zoological interchange which had, as 
a result, the formation of a mixed fauna, whose origin has hitherto 
been a little inexplicable. Passing from the upper part of this 
recently-upheaved land, and describing a complete circle through 
time and space, there returned to Argentina many of ‘the forms 
which had lived there during the Cretaceous period, but all of them 
modified and disguised. There emigrated at this epoch from North 
to South America the Mastodons, which had become extinct on the 
plains of the Pampa when, long geological periods previously, their 
forefathers the Pyrotheria disappeared from our land. With the 
Mastodons came the dogs, the felines, and the other carnivores 
descended from the ancient Sparassodonts, the llamas and the deer, 
the horses and the tapirs, which lived and multiplied on the 
Argentine plains by the side of the Toxodons, the Glyptodons and 
the Megatheria. But passing across these same lands the Argentine 
fauna advanced to the north and invaded North America. The 
clumsy Yoxodon of our land was exterminated in Nicaragua. The 
heavy Glyptodons of the Pampa wandered away as far as Anahuac, 
where their carapaces are found on the slopes of the valley of 
Mexico in the neighbourhood of the city of the same name, and still 
further to the north in the surface deposits of the plains of Texas. 
The carpincho (Hydrochoerus) of the River Parana wandered as far 
as Florida accompanied by the Chlamydotheriwm, the most robust of 
the true armadillos which lived in our land. The gigantic extinct 
sloths of the Buenos Aires plains, the Mylodons and the Megatheria, 
advanced to a still greater distance, their remains being met with 
