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TEXTBOOK OF ZOOLOGY 



Classification 



Birds constitute a relatively uniform group of animals. They are 

 much more similar throughout the class than any other group to be 

 studied. The differences between the most dissimilar birds are no 

 greater than the differences seen among frogs in the order Salientia, 

 in class Amphibia. Such characteristics as size, color, wing develop- 

 ment, type of beak and feet, are the bases for distinguishing the 



Pig. 306. — African ostrich, Struthio camelus. (From Metcalf, Textbook of Economic 

 Zoology, published by Lea and Febiger.) 



orders of birds. There are four orders of flightless birds which are 

 sometimes placed in a group designated as subclass Ratitae. This 

 group includes such birds as ostriches, rlieas, cassowaries, and kiwis. 

 The sternum has no keel in any of these, the feathers are without 

 barbules, and the wings are either absent or reduced. All other birds 

 would be grouped in subclass Carinatae according to this plan. More 

 than 14,000 species of birds are classified into twenty-five orders. 



