xv. 8-9 LIZARDS 407 



In some lizards and all snakes, the eyelids are modified to form an 

 immovable transparent spectacle covering the cornea, with the loss of 

 the nictitating membrane. The adaptive significance of this, which is 

 foreshadowed in some lizards by the development of a window in the 

 lower eyelid, is not always clear, since although it is found in many 

 that burrow or live in sand, it also occurs in arboreal forms. 



9. Suborder Lacertilia 



The modern lizards show extensive adaptive radiation (Fig. 233) 

 and include terrestrial, arboreal, burrowing, and aquatic forms. The 

 majority are carnivorous but there are some herbivores. It is difficult 

 to say which of the twenty or so living families is the most primitive, 

 and the grouping of these into infraorders is a matter of some difficulty. 



The Gekkota contains the geckos and a small group of Australasian 

 limbless forms, the pygopodids. Geckos are mainly small nocturnal 

 and arboreal insectivorous lizards of warm climates, with ridged pads 

 on the toes and sharp claws that enable them to climb an almost 

 smooth surface. Some species have taken to living in houses. The 

 tree-gecko Ptychozoon has webs of skin on the limbs and along the 

 sides of the body, which perhaps act as a parachute to break its fall. 

 Many geckos live in colonies and unlike most lizards are extremely 

 vocal, making clicking and cheeping sounds. Their hearing is probably 

 acute. The endolymphatic ducts of the inner ear are greatly expanded 

 to form sacs in the neck containing calcareous deposits, but the func- 

 tional significance of this is obscure. Their eyes are, as a rule, covered 

 by a spectacle. Geckos are the only Squamata that lay hard-shelled 

 eggs, those of other forms being leathery in texture. 



The Iguania is a large group comprising the agamids, iguanids, and 

 chameleons. The first two include terrestrial, arboreal, and amphibious 

 types, sometimes of large size, and often furnished with crests, dew- 

 laps, expansible throat-fans and other appendages that play a part 

 in rivalry and courtship. The males are often brightly coloured and 

 the whole group is characterized by a visually dominant behaviour 

 pattern. In some arboreal forms, as in the chameleons, the sensory 

 parts of the nose and the organ of Jacobson are reduced. 



The agamids are found in the Old World and Australasia and in- 

 clude such well-known types as the oriental 'blood-sucker' (Calotes), 

 so-called because of the red colour of the throat, the spiny lizards 

 (Uromastix) of the north African and Indian deserts, and the Aus- 

 tralian frilled lizard (Chlamydosaarns) (Fig. 240). The Indo-Malayan 

 genus Draco has a large lateral web, supported by the ribs, which can 



