REPTILIA 303 



In Amniota, the allantois grows around the embryo as a stalked 

 vesicle^ which in reptiles, birds and monotremes lies close beneath the 

 egg shell and acts as a respiratory organ during the rest of the em- 

 bryonic period. It also receives excretory matters from the kidneys. 

 In the mammals above the monotremes, an important vascular 

 connection takes place between mother and fetus by means of the 

 allantois. This is called the placenta. The allantois becomes 

 attached to a definite region of the uterine wall and from it vascular 

 processes or villi arise so that the fetal and maternal blood vessels 

 come into close relationship with each other. Gills are no longer 

 necessary since the allantoic placenta functions in the respiration and 

 nutrition of the fetus. 



Classification 



Super-order i. Cotylosauria. Primitive fossil forms. 



Super-order 2. Chelonia. Living forms, including turtles and 

 tortoises. 



Super-order 3. Therapsida (Theromorpha) . Fossils linked with 

 mammals. 



Super-order 4. Sauropterygia. Fossil forms with a long neck. 



Super-order 5. Ichthyopterygia. Fossil aquatic forms. 



Super-order 6. Archosauria. Besides the primitive fossil Theco- 

 dontia, Pterodactyla and Dinosauria, the super-order in- 

 cludes one living connecting type, the Rhyncocephalian 

 Hatteria, and the living orders of Crocodilia and Squamata. 



Since only four orders of the Reptilia have living representatives, 

 we shall discuss these first, and defer the description of fossils to 

 the section on Fossil Relatives (page 330). 



Living Orders of Reptilia 



Chelonia. 

 Rhyncocephalia. 

 Crocodilia. 

 Squamata. 



Characteristics 



The reptiles have an amnion, an allantois, a horny skin, ossified 

 skeleton, two auricles, two ventricles with incomplete septum 

 (except in the Crocodilia), a single occipital condyle, are cold 

 blooded, breathe by lungs and have twelve cerebral nerves. 



