198 YERTEBRATA. 



Kecent. 



An osseous exoskeleton CHELONIA. 



No osseous exoskeleton. 

 No sockets for the teeth. 



No eyelids OPHIDIA. 



With eyelids SAURIA. 



Sockets for the teeth CROCODILIA. 



Extinct. 



Limbs natatory. 



No neck ICHTHYOPTERYGIA. 



A long neck SAUROPTERYGIA. 



Limbs not natatory. 



With molar teeth TIIERIODONTIA. 



Without molar teeth. 

 Teeth conical, numerous. 



Four strong, unguiculate limbs DINOSAURIA.. 



Fore limbs adapted for flying PTEROSAURIA. 



Teeth wanting, or with two tusks only ANOMODONTIA. 



Order I. OPHIDIA. (Serpents.) 



Body slender, cylindrical, covered with horny scales ; no visible 

 limbs. No eyelids. Mouth dilatable. No sacrum, sternum, or 

 pelvis. Vertebrae and ribs very numerous. Tongue bifid. 



All the bones composing the upper and lower jaws are movably 

 united by ligament or muscle, and not by cartilage ; the mastoid 

 bone is also movable, and the os quadratum often passes behind 

 the head. This peculiar structure allows the mouth to be enor- 

 mously dilated. The vertebra are procoelous and very numerous 

 [from 200 to 420]. The ribs, of which there are sometimes 300 

 pairs, are always free at the extremity ; assisted by the ventral 

 scales, they are the real organs of locomotion. The teeth are slender, 

 curved, and anchylosed to the bones to which they belong. There 

 are no eyelids, but the eyes are covered by the transparent epi- 

 dermis. There are no external ears. The anal cleft is trans- 

 verse. 



In most serpents there is one lung of great length, the other 

 rudimentary or aborted. The heart "agrees with other organs 

 in its elongate form." 



In venomous serpents there are two poison-fangs firmly fixed 

 to the upper maxillary bones. These may be moved backwards 

 or forwards, so that with either movement the fangs are raised or 

 depressed. The fang is a long conical tooth, either grooved ex- 



