336 CLASS KEPTILIA. 



which increases in size as the anterior limbs become more 

 developed, and then serves for the attachment of a correspondingly 

 greater number of ribs. Except in the Amphisbaenidae and a 

 few other lizards the tail is long. The limbless forms and those 

 with reduced limbs are by no means specially related to one 

 another. They turn up in many of the families and the loss or 

 reduction of the limbs seems to be associated with some special 

 habit of life, such as burrowing or living among stones and thick 

 vegetation. 



Most lizards have an upper and lower eyelid and a nictitating 

 membrane, but in the Amphisbaenidae and Geckonidae and some 

 jScincidae the eyelids are fused over the eye as in snakes 

 and there is a cavity lined by conjunctiva between them 



FIG. 183. Pygopus lepidopus (R6gne animal). 



and the cornea. In some Scincidae the centre of the 

 lower eyelid is transparent and can be raised over the eye 

 without hindering the sight. In chameleons the single eyelid 

 is circular, consisting of a muscular ring of skin with circular 

 opening. An exposed tympanic membrane is usually present, 

 but in Amphisbaenidae both it and the tympanic cavity are 

 absent, and in many lizards (Anguis, Anelytropidae, Chamae- 

 leontidae etc.) the tympanic membrane is covered by the skin or 

 absent. 



The integument of lizards resembles in its general features 

 that of snakes, but presents much greater variety. As a general 

 rule it is scaly. The scales consist of horny epidermal 

 plates placed on dermal papillae and frequently overlap. 



