10 



backward and overlap each other like the shingles on a house. 

 While presenting a more or less firm and smooth exterior, their 

 lower portions are open and downy, thus combining lightness 

 with warmth, and forming a non-conducting medium admirably 

 adapted to conserve the heat of the body. The long feathers 

 forming the wings and tail are so arranged as to form exceed- 

 ingly light but effective instruments of flight, the wings present- 

 ing a smooth, strongly resisting surface to the air in the downward 

 stroke, while the tail serves the purpose of a rudder. The wings 

 are attached to the portion of the body affording the best leverage ; 

 the skeleton is constructed to afford the greatest possible strength 

 and firmness to the shoulder, and for the attachment of the 

 immense breast muscles that move the wings. For this purpose 

 the breast-bone or sternum is broad, long, and strongly keeled, 

 and all the bones entering into the scapular arch are firmly bound 

 together and to the thorax. The bones are hollow, admirably 

 combining lightness and strength, while air cavities are situated 

 in various parts of the body, to further lesson specific gravity. 

 The muscles which move the wings exceed in bulk all the other 

 muscles together, and sometimes nearly outweigh the remaining 

 portion of the body. Even the provision by which birds lay eggs 

 to be hatched by incubation has also reference to the function of 

 flight. If birds carried their young, as mammals do, until ready 

 to be born alive, their weight would so far impair the bird's 

 bouyancy as to greatly impede flight. 



As the anterior limbs of birds are exclusively specialized for 

 flight, the hind limbs are equally so for walking and grasping 

 being the only animals except man perfectly adapted to bipedal 

 locomotion. 



Subclasses. — Birds (Class Avcs) are divided into four sub- 

 classes, only one of which has living representatives, namely, the 

 Eurhipidiirce. 



Saururce. — The first Subclass, Saururce (lizard-tailed birds), 

 consists of the single genus Arc/icBopteryx, known as yet only from 

 a very few specimens discovered in the lithographic slate of 

 Solenhofen, Bavaria, a formation belonging to the Upper Jura. 

 It was first made known from a single feather, found in 1861, by 



