432 MANUAL OF ZOOLOGY SECT. 



the Vipers and Rattlesnakes, Pythons, Boas, Sea- 

 snakes, etc.). 



2. The Rhynchocephalia, including only the New 

 Zealand Tuatara (Hatteria). 



3. The Chelonia, including the Land Tortoises, Soft 

 Tortoises, River and Marsh Tortoises, and the Turtles. 



4. The Crocodilia, including the Crocodiles, Gavials, 

 Alligators and Caimans. 



The most striking external differences between a typical 

 Lizard (Fig. 193) and the Frog are in the covering of scales 

 in the case of the former, the comparative smallness of its 

 head and the presence of a distinct neck, the great length 

 of the caudal region, the shortness of the limbs, and the 

 approximate equality in length of the anterior and posterior 

 limbs. The anterior limbs are situated just behind the 

 neck, springing from the trunk towards the ventral surface. 

 The fore-limb, like that of the Frog, is divided into three 

 parts, the upper-arm or brachittm, the fore-arm or anti- 

 brachium, and the hand or manus ; there are five digits 

 provided with horny claws, the first digit or pollex being 

 the smallest. The hind-limbs arise from the posterior end 

 of the trunk towards the ventral aspect ; each, like that of 

 the Frog, consists of three divisions thigh or femur, shank 

 or cms, and foot or pes. The pes, like the manus, termin- 

 ates in five clawed digits, of which the first or hallux is the 

 smallest. The head is somewhat pyramidal, slightly de- 

 pressed : the openings of the external nares are situated 

 above its anterior extremity. The mouth is a wide slit- 

 like aperture running round the anterior border of the head. 

 At the sides are the eyes, each provided with upper and 

 lower opaque movable eyelids and with a transparent third 

 eyelid or nictitating membrane, which, when withdrawn, lies 

 in the anterior angle of the orbit. Behind the eye is a 



