DIVISIONS OF REPTILES. 397 



venous takes place as in the lower Reptilia, though probably 

 not to so complete an extent. It is this peculiarity of the 

 circulation in all Reptiles which conditions their low tempera- 

 ture, slow respiration, and generally sluggish vital actions. 



The lungs in all Reptiles, except the Crocodiles, are less 

 completely cellular than in the Birds and Mammals, and they 

 often attain a very great size. In no Reptile is the cavity of 

 the thorax shut off from that of the abdomen by a complete 

 muscular partition or " diaphragm ; " though traces of this 

 structure are found in the Crocodiles. The lungs, therefore, 

 often extend along the whole length of the thoracico-abdominal 

 cavity. In no case are the lungs connected with air-receptacles 

 situated in different parts of the body; and not uncommonly 

 there is only a single active lung, the other being rudimentary 

 or completely atrophied (Ophidia). 



Lastly, all Reptiles are essentially oviparous, but in some 

 cases the eggs are retained within the body till the young are 

 ready to be excluded, and the animals are then ovo-viviparous. 

 The egg-shell is usually parchment-like, but sometimes contains 

 more or less calcareous matter. 





CHAPTER LXIII. 



DIVISIONS OF REPTILES. 



CHELONIA AND OPHIDIA. 



THE class Reptilia is divided into the following nine orders, 

 of which the first four are represented by living forms, whilst 

 the remaining five are extinct : 



i. Chelonia (Tortoises and Turtles). 





2. Ophidia (Snakes). 



3. Lacertilia (Lizards). 



4. Crocodilia (Crocodiles and Alligators). 



5. Ichthyopterygia. 



6. Sauropterygia. 



Recent. 



Anomodontia. 



Pterosauria. 



Deinosauria. 



Extinct. 



ORDER I. CHELONIA. The first order of living Reptiles is 

 that of the Chelonia, comprising the Tortoises and Turtles, and 



