21f) Illinois State Lnhoratory of Natural History. 



CLASS KEPTILIA. 



Exoskeleton in the form of horny scales or bony plates. 

 One occipital condyle. Mandible present, each ramus of sev- 

 eral bones. Vertebrae without terminal epiphyses. No mam- 

 mary glands. Generally no diaphragm (an incomplete dia- 

 phragm is present in crocodiles). Respiration always by means 

 of lungs, sometimes aided by the walls of the pharynx. Heart 

 generally with three, sometimes with four, chambers. Two 

 aortic arches. Blood not warm; red corpuscles nucleated. 

 Alimentary canal terminating in a cloaca. Oviparous or ovo- 

 viparous. 



Body enclosed in a bony shell, wide, and more or less de- 

 pressed. Legs four. Turtles Order Chelonia. 



Body more or less cylindrical, never greatly depressed, covered 

 with small scales, generally imbricated. Eyelids and ex- 

 ternal ears present. Legs commonly four; if wanting, with 

 rudimentary sternal arch. Lizards Order Saukia. 



Body very long and slender, cylindrical, back covered with 

 small imbricated scales, belly commonly with larger scales. 

 No legs, or at most with rudiments of the hind pair. 

 Sternal arch, eyelids, and external ears wanting. Snakes. 



Order Ophidia. 



ORDER CHELONIA. 



Body enclosed between two shields (carapace, upper, and 

 plastron, lower) consisting of bony plates. Dorsal vertebrae 

 and ribs immovably united with the carapace. Bones of head 

 firmly united. Jaws covered with bony plates. No teeth. 

 No external auditory organs. Eyes with a nictitating mem- 

 brane. Four well-developed limbs. Oviparous. 



This well-defined group is represented in Illinois by a 

 rather small number of species. Our streams and lakes, more 

 especially the Illinois and Mississippi Rivers, with their exten- 

 sive sloughs, their numerous sandy shores suited to the process 

 of oviposition, and their abundance of animal and vegetable 

 life, would seem to form an ideal cheloniau habitat. About 



