xm PHYLUM CHORDATA 329 



represented only by mere vestiges ; and numerous intermediate 

 gradations exist between these and forms, such as Lacerta, with 

 well-developed limbs. The limbless Lizards (Fig. 989) bear a very 

 close resemblance to the Snakes, not only in the absence of the 

 limbs, but also in the general form of the body and the mode of 

 locomotion. 



The body of a Snake is elongated, narrow and cylindrical, 

 usually tapering towards the posterior end, sometimes with, more 

 usually without, a constriction behind the head. In the absence 

 of limbs the beginning of the short caudal region is only indicated 

 by the position of the cloacal opening. The fore-limbs are never 

 represented even by vestiges ; in some Pythons there are in- 

 conspicuous vestiges of hind-limbs in the form of small claw-like 

 processes. The mouth of the Snake is capable of being very 

 widely opened by the free articulation of the lower jaw, and it is 



FIG. 989. Pygopus lepidopus, with scale-like vestiges of hind-limbs. (After Brehm.) 



this mainly which distinguishes it from the snake-like Lizards. 

 But other, less conspicuous, points of distinction are the absence of 

 movable eyelids in the Snake, and also the want of a tympanum. 



Sphenodon or Hatteria, the New Zealand Tuatara (Fig. 990), the 

 only living representative of the Rhynchocephalia, is a Lizard-like 

 Reptile with a well-developed laterally-compressed tail, and 

 pentadactyle extremities, very similar to those of a typical Lizard. 

 The upper surface is covered with small granular scales, and a 

 crest of compressed spine-like scales runs along the middle of the 

 dorsal surface. The lower surface is covered with transverse rows 

 of large squarish plates. 



In the Chelonia (Fig. 991) the body is short and broad, enclosed 

 in a hard " shell " consisting of a dorsal part or carapace and a 

 ventral part or plastron. These are in most cases firmly united, 

 apertures being left between them for the head and neck, the 

 tail and the limbs. The neck is long and mobile ; the tail short. 



