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 Toxodon 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 Paver Parana wandered as far 

 as Florida accompanied by the Ohlamydothermm, the most robust of 

 the true armadillos which lived in our land. The snR'antic extinct 

 sloths of the Buenos Aires plains, the Mylodons and the Megatheria, 

 advanced to a still greater distance, their remains being met with 



