VER TEBRA TES. 3 



ness of their bones, by the tendency to fusion in many 

 parts of the skeleton, e.g., backbone and ankle, by the 

 absence of teeth in modern forms, by the fixedness of the 

 lungs and their association with numerous air sacs, and 

 so on. 



But here again different grades must be distinguished — 

 (i) there is the vast majority — the flying birds, who have a 

 breast-bone keel or carina, to which the muscles used in 

 flight are in part attached (Carinats) ; (2) there is the 

 small minority of running birds (ostriches, emu, cassowary, 

 and kiwi), whose wings are incapable of flight, and who 

 have no keel (Ratitge) ; and (3) there is an extinct type, 

 Archaopteryx, 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. More- 

 over, when we consider the extinct Mammals and Birds, 

 we perceive other resemblances linking the two highest 

 classes of animals to Reptiles. 



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 distinct 

 types : — the crocodilians, the unique New Zealand " lizard " 

 {Ha/teria), 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 a 

 practically or really four chambered heart, in never having 

 gills, and in having during embryonic life two important 

 "fcetal membranes," known as the amnion and the allantois. 



Amphibians, 



The Amphibians, such as frogs and newts, were once 

 regarded — e.g., by Cuvier — as naked Reptiles, but a more 

 accurate classification has linked them rather to the P'ishes. 



