22 HISTORICAL GEOLOGY 



have fins. 4. Reptiles (e.g. Snakes and Crocodiles), though in 

 many ways like Amphibians, never have gills, and always have 

 scales or bony plates developed from the skin. They are the most 

 highly organized cold-blooded animals. 5. Birds are plainly dis- 

 tinguished from all other animals by their covering of feathers. 

 They are too familiar to need special description. They are warm- 

 blooded creatures with well-developed heart and circulation of 

 blood. 6. Mammals (e.g. Dog and Man) include the highest of 

 all organisms, a characteristic being that they all suckle the young. 

 They are mostly quadrupeds, covered with hair, and dwellers on 

 land. The Whale is an exceptional mammal. 



Vertebrate fossils are common and of special interest because 

 they show the development of the higher animals. The simplest 

 (Ostracoderms) have been found in rocks of Ordovician age 

 (see below), and higher and higher forms were gradually intro- 

 duced and developed until the most complex Mammals appeared 

 in comparatively recent time. 



