The Tetrapoda: Amphibia, Reptiles, Birds 603 



kidney than the amphibians. Their new metanephric kidney is capable 

 of handling a much greater volume of metabolic wastes. This 

 perhaps is not very important in animals such as the reptiles, but 

 becomes an absolute necessity in their homoiothermic descendants, 

 the birds and mammals. 



For support on land, the reptiles have heavy, completely ossified 

 skeletons and well-developed legs capable of supporting the body. These 

 legs have been secondarily lost in the snakes and some lizards. Typi- 

 cally there are five toes equipped with claws which are useful for 

 clinging to a substratum and for rapid movements on land. Some liz- 

 ards have increased their speed of running by using only their hind 

 legs, thus they have become bipedal. This tendency is seen in several 

 groups, including the ancient dinosaurs. The well-developed skull 

 is equipped with a single occipital condyle. 



Classification of the Class Reptilia. — This class is divided into 

 a number of different subclasses and orders. Inasmuch as the fossil 

 forms are important in understanding the relationships of the modern 

 forms, they are included here. 



Subclass 1. Anapsida. 



Order 1. Cotylosauria. Extinct stem reptiles'. Examples: Seymour ia, Cap- 



torhiniis, Diadectes. 

 Order 2. Chelonia. Example : the turtles. 



Subclass 2. Ichthyopterygia. 



Order 1. Ichthyosauria. Examples: extinct ichthyosaurs which were highly 

 modified for a marine existence. 



Subclass 3. Synaptosauria. 



Order 1. Sauropterygia. Example: Extinct plesiosaurs which lived during 

 the Mesozoic and swam by means of powerful paddles. 



Subclass 4. Lepidosauria. 



Order 1. Eosuchia. Example: Extinct forms of the Permian and Triassic, 



ancestral to snakes and lizards. 

 Order 2. Rhynchocephalia. Example : living Sphenodon of New Zealand 



and related though extinct forms. 

 Order 3. Squamata. Examples : living lizards and snakes. 



Subclass 5. Archosauria, the "ruling reptiles." 



Order 1. Thecodontia. Example: extinct forms which were ancestors of 



dinosaurs, birds, and others. 

 Order 2. Crocodilia. Examples : modern crocodiles and alligators, only 



survivors of archosaurs. 

 Order 3. Pterosauria. Example : extinct flying reptiles, the pterosaurs, which 



possessed membranous wings. 



