18971 THE SOURCE OE THE TERTIARY MAMMALIA 257 



continent had not appeared, except as a few small islands. North 

 America, completely separated from South America, formed a great 

 island, with large lakes of brackish water ; and this part of South 

 America had lost its insular and peninsular form. The Argentine 

 Territory had completely emerged, and extended to the east towards 

 South Africa, while to the south and west it was prolonged to form 

 a large continent, which placed it in connection with Australia and 

 New Zealand. 



It was during this epoch in that great southern continent, and 

 especially in its central portion now constituting the Argentine 

 Territory, that the highest organisms developed, the great class of 

 the mammals which immediately spread over the southern lands, 

 and subsequently penetrated by different routes into the northern 

 hemisphere. 



The great barrier of the Andes was then low, and did not hinder 

 the atmospheric currents. The climate was hot and humid, and a 

 luxuriant vegetation covered all the Argentine Territory. As far as 

 the present Patagonian plains, to-day dry and sterile, there flourished 

 large forests of palms and conifers, whose petrified remains fill whole 

 deposits, in which one continually finds huge tree trunks trans- 

 formed into flint still occupying their natural position and constitut- 

 ing dead forests, forests of stone, columns of flint such as that which 

 one can see opposite the Museum of La Plata crowned with the bust of 

 the unfortunate Crevaux, and which the imagination of the dwellers 

 of the Patagonian deserts, on account of the undulation of the land, 

 takes to be the masts of petrified ships. 



Alternating with the branches and tree trunks transformed into 

 stone, which fill the deposits of sandy rock appearing at various 

 points of the Patagonian Territory, large bones are met with 

 similarly petrified, belonging to terrestrial vertebrates of the ex- 

 tinct group Dinosauria. They were reptiles with an enormously 

 thick tail, and the hind limbs much longer and thicker than the 

 fore limbs, so that, supporting the body on the hind limbs and tail, 

 they could assume a semi- vertical or oblique position resembling 

 that of a kangaroo. 1 When one says that as a matter of fact they 

 could have looked over the roofs of most of the buildings at La 

 Plata, one can judge of the truly colossal size which some repre- 

 sentatives of this group attained. 



The birds of that time were no less noteworthy than the reptiles. 

 They were such as Physornis and Phororhacos, true monsters, bipeds 

 with short and thick wings, the claws of an eagle, and the beak of 



1 Of the three sub-orders into which the Dinosauria are divided, namely, Sauropoda, 

 Theropoda, and Orthopoda, the characters mentioned above are peculiar to the two last. 

 The sub-order of the Sauropoda, to which the gigantic genera of Patagonia, Argyromurus 

 and Tilanosaurus Lyd. , belong, have the four limbs more or less equal, or the front pair 

 scarcely any shorter than the hind pair. 



