2.38 GIGANTIC TERRESTRIAL SAURIANS. 
mechanism will best be explained by reference 
to the figm-es in PL 23.* 
In the structure of these teeth, (PL 23, 
Figs. 1, 2, 3), we find a combination of 
mechanical contrivances analogous to those 
which are adopted in the construction of the 
knife, the sabre, and the saw. When first pro- 
truded above the gum, (PL 23, Figs. 1'. 2.) the 
apex of each tooth presented a double cutting- 
edge of serrated enamel. In this stage, its 
position and line of action were nearly vertical, 
and its form like that of the two-edged point 
of a sabre, cutting equally on each side. As 
the tooth advanced in growth, it became curved 
* The outer margin of the jaw (PI. 23, Fig. V. 2'.) rises 
nearly an inch above its inner margin, forming a continuous la- 
teral parapet to support the teeth on the exterior side, where the 
greatest support was necessary; whilst the inner margin (PI. 23, 
Fig. 1') throws up a series of triangular plates of bone, forming 
a zig-zag buttress along the interior of the alveoli. From the 
centre of each triangular plate, a bony partition crosses to the 
outer parapet, thus completing the successive alveoli. The new 
teeth are seen in the angle between each triangular plate, rising 
in reserve to supply the loss of the older teeth, as often as pro- 
gressive growth, or accidental fracture, may render such renewal 
necessary ; and thus affording an exuberant provision for a rapid 
succession and restoration of these most essential implements. 
They were formed in distinct cavities, by the side of the old teeth, 
towards the interior surface of the jaw, and probably expelled 
them by the usual process of pressure and absorption ; insinuat- 
ing themselves into the cavities thus left vacant. This contri- 
vance for the renewal of teeth is strictly analogous to that which 
takes place in the dentition of many species of existing Lizards. 
