REPTILES. 293 



The living reptiles in their external form present three 

 types : (i}, the quadrupedal long-tailed form represented by the 

 lizards and alligators ; (2), the cuirassed forms of the turtles ; 

 and (3), the apodal forms of the snakes and footless lizards. 

 If the fossil groups also be taken into consideration the range 

 of shape is still greater ; for it includes not only the swimming- 

 groups, the plesiosaurs and ichthyosaurus, but the flying reptiles, 

 the pterodactyls. 



A few of the fossil forms apparently had naked skins ; but 

 in the rest the body is more or less completely covered by 

 scales, which differ from those of the ichthyopsida, in that they 

 are cornifications of the superficial layers of the epidermis. 

 These are re-enforced in many by dermal ossifications, which may 

 be minute as in certain lizards, or larger scutes, as in the croco- 

 diles and in many extinct groups ; whereas in some fossil croc- 

 odiles (Teleosaurus) and dinosaurs, they form a complete armor 

 for the body. In the turtles this formation of armor reaches 

 its extreme, for here the dermal 

 plates are usually united with the 

 ribs to form a firm carapace and 

 plastron. Usually there is jio 

 pigment in the epidermis ; but the 

 derma contains pigment cells, 

 which in certain lizards (Ano/is, 



Chamdco) are Capable of produ- FIG. 292. Section and medial view 



of jaw of Anohs, showing pleurodont 



cing marked color changes under dentition. 



control of the nervous system. 



Epidermal glands are rare. In some turtles scent glands occur 



beneath the mandibles or on the side of the plastron ; in the 



snakes and crocodiles similar glands are connected with the 



cloaca ; while in most lizards there is a row of glands on 



the ventral surface of the femur. 



Teeth (lacking in turtles and some pterodactyls and anomo- 

 dontia) are usually restricted to the premaxillary, maxillary, and 

 dentary bones ; but in snakes and some lizards they may also 

 occur upon the palatines and pterygoids. These tegth. are 

 usually simple, without folding of enamel, and only in the therio- 

 dontia are they differentiated into incisors, canines, and molars. 



