530 MANUAL OF ZOOLOGY. 



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 LX. 



DIVISIONS OF REPTILES. 



CHELONIA AND OPHIDIA. 



THE class Reptilia is divided into the following ten orders, 

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

 the remaining six are extinct : 



1. Chelonia (Tortoises and Turtles). ^ 



2. Ophidia (Snakes). V Recent 



3. Lacertiha (Lizards). ( 



4. Crocodilia (Crocodiles and Alligators). ) 



5. Ichthyopterygia. 



6. Sauropterygia. 



7. Anomodontia. ^ Extinct 



8. Pterosauria. 



9. Deinosauria. 

 i o. Theriodontia. 



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

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

 distinguished by the following characters: There is an osseous 

 exoskeleton ivhich is combined with the endoskeleton to form a 

 kind of bony case or box in which the body of the animal is 

 enclosed, and which is covered by a leathery skin, or, more 

 usually, by horny epidermic plates. The dorsal vertebra, with 



