xii PHYLUM CHORDATA 435 



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 

 pairs. 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 brachium, 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 crus, and foot or pes. The pes, like the manus, termi- 

 nates 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 the 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 eyelid or nictitating membrane, which, when withdrawn, lies 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 

 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 covering 

 all parts, differing in size in different positions. 



