270

MEGALOSAURUS.

adapted to the destructive office for which they have been designed.Their form and mechanism will be best explained by reference to thefigures.

“ The outer margin of the jaw (PI. 70, fig. 8, b ) rises nearly aninch above its inner margin, forming a contiguous lateral parapet tosupport the teeth on the exterior side, where the greatest support wasnecessary, whilst the inner margin throws up a series of triangularplates of bone (c, c,) forming a zig-zag buttress along the interior ofthe alveoli. From the centre of each triangular plate, a bony par-tition crosses to the outer parapet, thus completing the successive alveoli.The new teeth {a, a) are seen in the angle between each triangularplate, rising in reserve to supply the loss of older teeth, as often asprogressive growth, or accidental fracture, may render such renewalnecessary, and thus affording an exuberant provision for a rapid suc-cession and restoration of these most essential implements. Theywere formed in distinct cavities, by the side of the old teeth, towardsthe interior surface of the jaw, and probably expelled them by theusual process of pressure and absorption, insinuating themselves into thecavities thus left vacant. This contrivance for the renewal of teeth, isstrictly analogous to that which takes place in the dentition of manyspecies of existing lizards.

“ In the structure of these teeth (PI. 70, figs. 9 & 10), we find acombination of mechanical contrivances analogous to those which areadopted in the construction of the knife, the sabre, and the saw.When first protruded above the gum, the apex of each tooth presenteda double cutting edge of serrated enamel. In this stage, its positionand line of action were nearly vertical, and its form, like that of thetwo-edged point of a sabre, cutting equally on each side. As thetooth advanced in growth, it became curved backwards, in the formof a pruning-knife, and the edge of serrated enamel was continueddownwards to the base of the inner and cutting side of the tooth,whilst on the outer side a similar edge descended, but to a short dis-tance from the point (fig. 10) and the convex portion of the toothbecame blunt and thick, as the back of a knife is made thick, for thepurpose of producing strength. The strength of the tooth w r asfurther increased by the expansion of its sides (as represented in the