190 GIGANTIC TERRESTRIAL SAURIANS. 



Whilst the crown of the tooth was thus gradually dimi- 

 nishing above, a simultaneous absorption of the root M'ent 

 on below, caused by the pressure of a new tooth rising to 

 replace the old one, until by this continual consumption at 

 both extremities, the middle portion of the older tooth was 

 reduced to a hollow stump, (Figs, 10, 11,) which fell from 

 the jaw to make room for a more efficient successor.* In 

 this last stage the form of the tooth had entirely changed, 

 and the crown had become flat, like the crown of worn out 

 human incisors, and capable of performing imperfect masti- 

 cation after the cutting powers had diminished. There is, I 

 believe, no other example of teeth which possess the same 

 mechanical advantages as instruments of cutting and tear- 

 ing portions of vegetable matter from tough and rigid plants. 



of the crown was thus perpetually maintained with a sharp cutting edge in 

 front, like that of the nippers. (See Figs. 7. 8. 12.) 



The younger tootli, (Fig. 1,) when first protruded, was lancet-shaped, 

 with a serrated edge, extending on each side downwards, from the. point to 

 its broadest portion, as in the living Iguana. (Pi. 24./ 13, and Fig. 4 ) This 

 serrature ceased at the broadest diameter of the tootii, i. e. precisely at the 

 line, below which, had they been continued, they would have had no effect 

 in cutting. (Pi. 24. / 2. 6. 8. 9. 12.) As these saws were gradually worn 

 away, the cutting power was transferred to the enamel in front, and here we 

 find a provision of another kind to give efficacy and strength. The front 

 was traversed longitudinally by alternate ridges and furrows, (PI. 24, Figs. 

 2, 5, 6, 7, 8,) the ridges serving as ribs or buttresses to strengthen and pre- 

 vent the enamel from scaling off, and forming, together with the furrows, 

 an edge slightly wavy, and disposed in a series of minute gouges, or fluted 

 chisels; hence the tooth became an instrument of greater power to cut tough 

 vegetables under the action of the jaw, than if the enamel had been in a con- 

 tinuous straight line. By these contrivances, also it continued effective 

 during every stage through which it passed, from the serrated lancet-point of 

 the new toolh, (Fig. 1,) to its final consumption. (Figs. 10, 11.) 



* In PI. 24, Fig. 13, the jaw of a recent Iguana exhibits the commence- 

 ment of this process, and a number of young teeth are seen farcing their 

 way upwards, and causing absorption at the base of the older teeth. Figs 

 10, 11, exhibit the effect of similar absorption upon the residuary stump of 

 the fossil tooth of an Iguanodon. 



