4 o8 TROPICAL AMERICA AND ITS ANIMALS 



and are further distinguished by the longer wings, the fully feathered head and 



neck, and the absence of a tail. On the other hand, they resemble their African 



relatives in the superior size of the cock, as compared with the hen, and likewise 



in the presence of after-shafts to the body-feathers. 



From their large size, rheas are the most distinctive birds of the campos, or 



plains, of Brazil and the pampas of Argentina. It has been considered that these 



ostrich-like birds form an essentially southern group ; but against this view is the 



fact that they are represented in the upper Tertiary deposits of northern India, 



as well as in the lower Tertiaries of Egypt. 



Among South American reptiles particular interest attaches to 

 River-Tortoises. . ° r r . 



the river-tortoises, or terrapins, all or which belong to the group in 

 which the head and neck are moved sideways in place of being retracted with the 

 S-like flexure characteristic of the tortoises and terrapins of the Northern 

 Hemisphere. The largest is the great aru tortoise of the Amazon (Podocnemis 

 expansa), whose shell may be close on a yard in length. These tortoises are taken 

 by the natives for their flesh, while their eggs yield a kind of oil. The matamata 

 (Chelys fimbriata), which has irregular processes of skin on its head and neck and 

 a much corrugated shell, is the sole representative of its genus. Podocnemis, on 

 the other hand, occurs elsewhere in Madagascar at the present day, while it is also 

 represented in the Tertiary formations of the Northern Hemisphere, a circumstance 

 indicating apparently that the Pleurodira, as the side-necked tortoises are technically 

 termed, originated in the north, and made their way southwards during the 

 Tertiary epoch. 



None of the families of these tortoises is peculiar to South America, the 

 Pelomedusidce being represented at the present day in Ethiopian Africa and 

 Madagascar by the genera Pelomedusa and Sternothcsrus, and in South America 

 and Madagascar by the above-mentioned Podocnemis. 



The second family, Chelyidce, on the other hand, is partly South American 

 and partly Australasian ; the American genera being the above-mentioned Chelys, 

 together with Hydromedusa, represented by one species from Brazil and a second 

 from southern Brazil and Argentina, Hydrastis with some seven species, Platemys 

 with two, and Rhinemys with a single representative. The Australasian genera, 

 which range into New Guinea, are three in number. 



The absence of fresh- water tortoises of the family Trionychidce from South 

 America is a fact in geographical distribution almost as important as the presence 

 of the aforesaid side-necked tortoises, especially when it is borne in mind that the 

 former group is well represented in North America. 



crocodiles and Crocodiles are represented in South America by the sharp-nosed 



Caimans. Crocodilus americanus, while the caimans (Caiman), which differ 

 from alligators in having bony plates on the lower surface of the body, are peculiar 

 to the region. Caimans, frequently called alligators, include five species, of which 

 the black caiman, or black jacare, is the largest, growing to a length of about 

 13 feet. In colour, it is black above and yellow beneath ; and it is further 

 characterised by the upper eyelid being flat and finely striated, with a small bony 

 plate on the inner side. In another species, C. solerops, the upper eyelid is 

 prominent and tuberculated, one of the tubercles sometimes forming a small horn. 



