BIRDS 



(6) Metatheria, Didelphia, or Marsupials — the prematurely bearing, 

 usually pouch-possessing kangaroos, opossums, etc. 



(c) Prototheria, Ornithodelphia, or Monotremes— the egg-laying 

 duckmole (Ornithorhynchus), Echidna, and Proechidna. 



Fig. 2. — Phenacodus, a primitive extinct Mammal. — After Cope. 



Birds. — There can be 

 no hesitation as to the 

 class which ranks next to 

 Mammals. For Birds are 

 in most respects as highly 

 developed as Mammals, 

 though in a different direc- 

 tion. They are character- 

 ised by their feathers and 

 wings, and many other 

 adaptations for flight, by 

 their high temperature, 

 by the frequent spongi- 

 ness and hollowness of 

 their bones, by the tend- 

 ency to fusion in many 

 parts of the skeleton, 

 by the absence of teeth 

 in modern forms, by the 

 fixedness of the lungs 

 and their association with 

 numerous air sacs, and so on. 



Fig. 3. — Extinct moa and modern 

 kiwi. — After Carus Sterne. 



But here again different grades must be distinguished — (i) There is 

 the vast majority — the flying birds, with a breast -bone keel or carina, to 



