REPTILES 237 



lizards, as was formerly the practice. The large head is fairly- 

 well marked off from the stoutish body, which again passes 

 into a large flattened tail. The short limbs present the usual 

 regions, and each of the extremities possesses five clawed digits. 

 The eyes are large, but there is no external trace of organs of 

 hearing. The scales of the upper surface are in the form of small 

 granules, with the exception of a series of sharp spiny ones which 

 make up a crest running down the middle line. A number of 

 squarish horny plates arranged in transverse rows cover the 

 under surface. 



As regards its internal structure the Tuatara differs in many 

 important points from typical lizards, but as the distinctions are 

 largely of a technical nature, only a few of them can be men- 

 tioned here. 



The quadrate bone, to which the lower jaw is hinged, is not 

 movable as in a lizard (see p. 206), but firmly fixed to the skull 

 as in the crocodile, and the two halves of the lower jaw are 

 united together in front only by fibrOus tissue. The centra of 

 the vertebrcz are biconcave in shape (see p. 221), and exhibit 

 other primitive characters. The ribs possess uncinate processes 

 as in Birds and Crocodiles (see p. 145), and there are abdominal 

 ribs as in the latter (see p. 206). The teeth are also peculiarly 

 arranged, there being two rows above, one on the upper jaw and 

 the other on the roof of the mouth, and into the groove between 

 these the teeth of the lower jaw bite, getting in course of time 

 ground to a sharp edge. In the front of the upper jaw are two 

 wedge-shaped teeth, almost reminding one of those present in 

 a rabbit. All these teeth are firmly fused with the edges of 

 the firm bones, and in old animals may be so much ground 

 down that the bones themselves act as biting organs. 



