DINOSAUEIA 259 



In the Megalosaurus the greater relative development of 

 the inner alveolar wall, as compared with the dentigerous part 

 of the jaw in existing Sanrians, narrows the dental groove, and 

 covers a greater proportion of the bases of the teeth, besides 

 concealing more or less completely the germs of their suc- 

 cessors. Moreover, instead of the mere shallow impressions 

 upon the inner side of the outer alveolar plate to which the 

 teeth are attached in modern lizards, there are distinct sockets 

 formed by bony partitions connecting the outer with the inner 

 alveolar wall in the jaw of the Megalosaurus. These parti- 

 tions rise from the outer side of the inner alveolar wall in the 

 form of triangular vertical plates of bone, and from the middle 

 of the outer side of each plate a bony partition crosses to the 

 outer parapet, completing the alveoli of the fully-formed or 

 more advanced teeth ; the series of triangular plates forming a 

 kind of zig-zag buttress along the inner side of those alveoli. 

 The outer parapet rises an inch higher than the inner one. 



Fig. 75 exhibits a portion of another jaw of the Megalo- 

 saurus, also from Stonesfield oolite, from which the inner wall 

 has been removed to show the germ of a successional tooth c, 

 about to succeed an old tooth «, which has been broken, and 

 near to which is a newly-formed tooth, b, coming into place. 

 These teeth will exemplify the shape of the crown of the- tooth, 

 which is subcompressed, slightly recurved, sharp edged, and 

 sharp-pointed, the edges being minutely serrated ; the edge 

 upon the convex or front border b becomes blunted as it 

 descends about two-thirds of the way towards the base of the 

 tooth ; that upon the concave hinder border a is continued to 

 the base. The lower half of the crown is thicker towards the 

 fore margin than towards the hind one ; so that a transverse 

 section, like that above, a, in fig. 75, gives a narrow oval form 

 pointed behind. At the upper half of the crown the sides 

 slope more equally from the middle thickest part to both 

 margins, and the section is a narrow pointed ellipse. The 



