62 Vertebrata. 



hearing are also largely developed in some birds, 

 notably in vultures and owls, the latter being pro- 

 vided with a distinct external ear. 



Most birds have but one oviduct, and that is on 

 the left side ; in its lining there are glands which 

 secrete the white of the egg, its membrane, and the 

 shell, during the downward passage of the yolk. The 

 embryonic bird is provided with a rudimental knob 

 on its pre-maxillary bones, which it uses in breaking 

 the egg-shell wherein it is contained. 



CHAPTER XII. 



CLASSIFICATION OF BIRDS. 



33. Primary Divisions. About 8,000 species or 

 birds are known to the naturalist, and these are divided 

 into two primary sub-classes. 



The first sub-class is called Ratidce, and includes 

 all those birds which have a sternum without a keel, a 

 rudimentary furculum and wings, feathers with discon- 

 tinuous barbs and not distributed in feather tracts, 

 and with no oil gland. They are all natives of warm 

 or temperate climates, and strictly limited in their 

 range. Several gigantic forms which existed until 

 recently, are now extinct, such as the moa of New 

 Zealand and the sepyornis of Madagascar. The 

 ostrich of Africa is a familiar instance, and is pro- 

 vided only with two toes. The American ostrich 

 or rhea is smaller, and inhabits South America. 



