248 GIGANTIC TERRESTRIAL SAURIANS. 
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 
imperlect mastication after the cutting powers 
had diminished. There is, I believe, no other 
example of teeth which possess the same me- 
chanical advantages as instruments of cutting 
and tearing portions of vegetable matter from 
tough and rigid plants. In this curious piece 
of animal mechanism, we find a varied adjust- 
ment of all parts and proportions of the tooth, 
to the exercise of peculiar functions ; attended 
by compensations adapted to shifting conditions 
of the instrument, during different stages of its 
and li 0 re wc 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 prevent the 
enamel from scaling off, and forming, together with the furrows, 
an edge slightly wavy, and disposed in a series of minute googes, 
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 continuous straight line. By these 
contrivances, also it continued effective during every stage 
through which it passed, from the serrated lancet-point of the 
new tooth, (Fig. 1), to its final consumption. (Fig. 10, 11.) 
* In PI. 24, Fig. 13, the jaw of a recent Iguana exhibits the 
commencement of this process, and a number of young teeth are 
seen forcing their way upwards, and causing absorption at the 
base of the older teeth. Figs. 1 0, 1 1 , exhibit the effect of simi- 
lar absorption upon the residuary stump of the fossil tooth of an 
Iguanodon. 
