DENTAL SYSTEM OF REPTILES. 251 



DENTAL SYSTEM OF REPTILES. 



Ill the class reptilia an entire order {Chelonia\ including 

 tlie tortoises, terrapenes, and turtles, are devoid of teeth ; 

 but the jaws in these edentulous reptiles are covered by a 

 sheath of horn, which in some species is of considerable 

 thickness and density; its working surface is trenchant in 

 the carnivorous species, but is variously sculptured and 

 adapted for both cutting and bruising in the vegetable 

 feeders. No species of toad possesses teeth ; neither have 

 the jaws the compensatory covering above described in 

 the chelonians. Frogs have teeth in the upper but not 

 in the lower jaw. Newts and salamanders have teeth in 

 both jaws, and also upon the palate; and teeth are found 

 in the latter situation as well as on the jaws in most ser- 

 pents and in the iguana lizard. In most other lizards and 

 in crocodiles, the teeth are confined to the jaws: in the 

 former they are cemented or anchylosed to the jaw ; in 

 the latter they are implanted in sockets. 



The existing lizards exhibit many modifications in the 

 form of the teeth, according to the nature of the food. 

 They are pointed with sharp cutting edges in the great 

 carnivorous monitor {Varanus\ and are obtuse and 

 rounded like paving-stones in the herbivorous or mixed 

 feeding scinks, called, on account of the shape of the 

 teeth, cyclodus. The gigantic extinct lizards showed 

 similar modifications of their teeth. The megalosaurus 

 had teeth which combined the properties of the knife, 

 the sabre, and the saw (Fig. 62). When first protruded 

 above the gum, the apex of the tooth presented a double 

 cutting edge of serrated enamel ; its position and line of 



