4 GENERAL SURVEY OF THE ANIMAL KINGDOM. 



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 

 which the muscles used in flight are in part attached (Cannatse; ; {2) 

 there is the small minority of running birds (ostriches, emu, cassowary, 

 kiwi, and extinct moa), with wings incapable of flight, and with no keel 

 (Ratitse) ; and (3) there is an extinct type, Archmopteryx, with markedly 

 reptilian affinities. 



Reptiles.— There are no close relationships between 

 Birds and Mammals, but the old-fashioned Monotremes 

 have some markedly reptilian features, and so have some 

 aberrant living birds, such as the Hoatzin and the Tinamou. 

 Moreover, when we consider the extinct Mammals and 

 Birds, we perceive other resemblances linking the two 

 highest classes to the Reptiles. 



Fig. 4. — Crocodiles. 



Reptiles do not form a compact class, but rather an 

 assemblage of classes. In other words, the types of Reptile 

 differ much more widely from one another than do the 

 types of Bird or Mammal. Nowadays there are five dis- 

 tinct types : — the crocodilians, the unique New Zealand 

 "lizard" {Hatteria), the lizards proper, the snakes, and the 

 tortoises. But the number of types is greatly increased 

 when we take account of the entirely extinct saurians, who 

 had their golden age in the inconceivably distant past. 



The Reptiles which we know nowadays are scaly-skinned 

 animals ; they resemble Birds and Mammals in having 

 during embryonic life two important "foetal membranes" 

 (the amnion and the allantois), and in never having gills ; 



