xii PHYLUM CHORDATA 433 



circular brown patch of skin the tympanic membrane 

 corresponding closely to that of the Frog, but somewhat 

 sunk below the general level of the skin. The trunk is 

 elongated, strongly convex dorsally, flatter at the sides and 

 ventrally. At the root of the tail on the ventral surface is 

 a slit-like transverse aperture the anus or cloacal aperture. 

 The tail is cylindrical, thick in front, gradually tapering to 

 a narrow posterior extremity ; it is nearly twice as long as 

 the head and trunk together. 



There is an exoskeleton of horny epidermal scales cover- 

 ing all parts, differing in size in different positions. 



In some groups of Lizards the tail is comparatively short 

 and thick ; and in others it is depressed and expanded into 

 a leaf-like form. In the Chamaeleons the long and tapering 

 tail is used as a prehensile organ, the coiling of which 

 round branches of the trees in which the animal lives aids 

 in maintaining the balance of the body in climbing from 

 branch to branch. 



In the limbs there is likewise a considerable amount of 

 variation in the different groups of the Lacertilia. Moder- 

 ately long pentadactyle limbs like those of Lacerta are the 

 rule. In the Chameleons both fore- and hind-limbs 

 become prehensile by a special modification in the arrange- 

 ment and mode of articulation of the digits. In these 

 remarkable arboreal Reptiles the three innermost digits of 

 the manus are joined together throughout their length by 

 a web of skin, and the two outer digits are similarly 

 united : the two sets of digits are so articulated that they 

 can be brought against one another with a grasping move- 

 ment somewhat analogous to the grasping movements of 

 a Parrot's foot or of the hand of Man. A similar arrange- 

 ment prevails in the pes, the only difference being that the 

 two innermost and three outermost digits are united. In 

 Man. Zool. F F 



