CHAPTER XXV 

 SUBPHYLUM VERTEBRATA, CLASS AVES 



Birds differ from all other vertebrates in possessing 

 feathers, though the scaly feet suggest relationship to 

 reptiles. Certain extinct species possessed teeth ,and had 

 separate toes ending in claws on the front limbs, but all 

 modern birds have the toothless jaws covered with a horny 

 beak and the digits of their wings are more or less rudi- 

 mentary. Birds, like mammals, are " warm-blooded "- 

 that is, are capable of maintaining a constant body 

 temperature. 



There are two subclasses of Aves: 



Subclass 1. Archseronithes.^ Extinct birds having toothed jaws, a 

 long lizard-like tail composed of separate vertebrae, and three separate, 

 clawed digits on each of the fore limbs. 



Subclass 2. Neornithes. Extinct and modern birds in which the 

 terminal vertebrae of the tail are fused to form a pygostyle, or "plow- 

 share" bone; the jaws are toothless, except in some extinct species; the 

 bones of the fore limb are more or less fused and reduced in size. 



SUBCLASS 1. ARCHvEORNITHES 



This group of birds is known from only two specimens 

 and a separate feather, found in the lithographic slate at 

 Solenhofen, Bavaria. All these fossils are referable to a 

 single species, Archceopteryxlithographica (Fig. 102). This 

 bird was about the size of a crow. It had a long flexible 

 tail with feathers along either side, and the wings bore 

 three free digits, each with a claw, while the hind legs had 

 four toes, like those of modern birds. The jaws were 

 armed with a row of strong teeth set in sockets. 



Though this curious flying animal, preserved as a fossil 

 for our inspection from Jurassic times, has characteristics 



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