Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/194

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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. 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.)