270 MEGALOSAURUS. 



adapted to the destructive office for which they have been designed. 

 Their form and mechanism will be best explained by reference to the 

 figures. 



" The outer margin of the jaw (PI. 70, fig. 8, b) rises nearly an 

 inch above its inner margin, forming a contiguous lateral parapet to 

 support the teeth on the exterior side, where the greatest support was 

 necessary, whilst the inner margin throws up a series of triangular 

 plates of bone ic,c,) forming a zig-zag buttress along the interior of 

 the 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 triangular 

 plate, rising in reserve to supply the loss of older teeth, as often as 

 progressive growth, or accidental fracture, may render such renewal 

 necessary, and thus affording an exuberant provision for a rapid suc- 

 cession 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, insinuating themselves into the 

 cavities thus left vacant. This contrivance for the renewal of teeth, is 

 strictly analogous to that which takes place in the dentition of many 

 species of existing lizards. 



" In the structure of these teeth (PL 70, figs. 9 & 10), 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 protruded above the gum, 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 backwards, in the form 

 of a pruning-knife, and the edge of serrated enamel was continued 

 downwards 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 tooth 

 became blunt and thick, as the back of a knife is made thick, for the 

 purpose of producing strength. The strength of the tooth was 

 further increased by the expansion of its sides (as represented in the 



