2 GUIDE TO REPTILES AND AMPHIBIANS. 



majority of which the young come into the world as aquatic animals 

 (" tadpoles "), breathing the air dissolved in water by means of gills, 

 but subsequently undergo a marked change (metamorphosis) into 

 the adult form, when atmospheric air is breathed by means of 

 lungs. It is true that in some cases the gill-bearing tadpole form 

 is retained throughout life (the creature breeding in this condition), 

 and also that in other instances the animal comes into the world 

 in the permanent air-breathing condition. In the latter case 

 the larval stages are passed through within the body of the 



Fig. 1. 



Back view of Skull of Crocodile, without the lower jaw. To show the single 

 knob, or "condyle" (o), by which the skull is articulated to the first 

 joint of the back-bone, or vertebral column ; and the quadrate-bone (q), 

 to the lower end of which the lower jaw would be attached. 



female parent or, more rarely, within the shell of an egg which is 

 laid (Csecilians). In existing Amphibians the skull is articulated to 

 the first vertebra by means of two knobs, or " condyles," as in 

 Mammals. 



At the present day Eeptiles and Amphibians are sharply distin- 

 guished from one another, and while the former show many decided 

 relationships to Birds (still more emphasised in some of their extinct 

 predecessors), the latter do not exhibit any such affinity. 



When, however, extinct Reptiles and Amphibians are taken into 



