252 



DENTAL SYSTEM OF KEPTILES. 



action were nearly vertical, and its form, like that of tlie 

 two-edged sword, cutting equally on 

 each side. As the tooth advanced in 

 growth, it became curved backwards 

 in the form of a pruning-knife, and 

 the edge of serrated enamel was con- 

 tinued downwards to the base of the 

 concave and cutting side of the tooth ; 

 whilst on the other side a similar 

 edge descended but a short distance 

 from the point, and the convex part 

 of the tooth became blunt and thick, 

 as the back of a knife is made thick 

 for the purpose of producing strength. 

 In a tooth thus formed for cutting 

 along its concave edge, each move- 

 ment of the jaw combined the power 

 of the knife and the saw. The back- 

 ward curvature of the full-grown 

 teeth enables them to retain, like barbs, the prey which 

 they had penetrated. 



In the iguanodon — the gigantic contemporary of the 

 megalosaurus — the crown of 

 the teeth (Fig. 63) was so shaped 

 that, after the apex became 

 worn down, it presented a 

 broad and nearly horizontal 

 surface, exposing dental sub- 

 stances of four different de- 

 grees of density, viz: a ridge 

 of enamel along the outer bor- 

 der of the crown ; a layer of 



NIC'.V-I'OIiMI'.r) AMI WOltN TEETH OF ' -^ 



THE luuAxoDox. liard or unvascular dentine 



TOOTH OF THE JIEGALO- 

 SAURUS. 



G3. 



