VERTEBKATA. 327 



them, that the generative products and the refuse of di- 

 gestion are all discharged through one common outlet. 



The sole organs of prehension are the beak and feet. 

 The circulation is double, as in Mammals, starting from a 

 four -chambered heart. Respiration is more complete 

 than in other Vertebrates. The lungs are fixed, and com- 

 municate with air-sacs in various parts of the body, as 

 along the vertebral column, and also with the interior of 

 many bones, as the humerus and femur, which are usu- 

 ally hollow and marrowless. 168 Both brain and cord are 

 much larger relatively than in Reptiles; the cranium is 

 larger in proportion to the face ; and the parts of the brain 

 are not situated in one plane, one behind the other. The 

 cerebrum is round and smooth, and the cerebellum single- 

 lobed. The ears resemble those of Crocodiles; but the 

 eyes are well developed, and protected by three lids. They 

 are placed on the sides of the head, and the pupil is al- 

 ways round. The sexes generally differ greatly in plu- 

 mage, in some cases more widely than two distinct species, 

 but the coloration of either sex of any one species is very 

 constant. 



There are two subclasses. 1 " 



SUBCLASS I. Ratitae (Cur sores). 



This small and singular group is characterized by hav- 

 ing no keel on the breastbone, rudimentary wings, feath- 

 ers with disconnected barbs, and stout legs. The African 

 Ostrich has two toes, the Cassowary three, and the Apte- 

 ryx four. 



Its representatives are the Ostrich (Struthio) of Africa 

 and Arabia, South American Ostrich (Rhea\ Cassowary 

 (Oasuarius) of the East Indian Archipelago and Austra- 

 lia, Emu (DromcGus) of Australia, and Apteryx, or Kiwi- 

 kiwi, of New Zealand. Besides these, there are extinct 

 gigantic forms from Madagascar (^pyornis) and from 



