Whilst the crown of the tooth was thus gradually diminishing
above, a simultaneous absorption of the root went
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.[1] 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 mastication
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 tearing
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 tooth, (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. (Pl. 24. f. 13, and Fig. 4.) This serrature ceased at the broadest diameter of the tooth, i. e. precisely at the line, below which, had they been continued, they would have had no effect in cutting. (Pl. 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, (Pl. 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 Pl. 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. 10, 11, exhibit the effect of similar absorption upon the residuary stump of the fossil tooth of an Iguanodon.
