4 GENERAL SURVEY OF THE ANIMAL KINGDOM. 
which the muscles used in flight are in part attached (Carinatz) ; (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 
(Ratitee) ; and (3) there is an extinct type, Avcheopteryx, 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. 
Fic. 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” (Sphenodon), 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 ; 
they differ from them in being “cold-blooded,” and in 
many other ways. 
