D1N0SAURIA. 287 



the alternating position of neural arch and centrum. The 

 articular surfaces of the free vertebrae are nearly flat ; the 

 neural arch develops a platform which in the anterior dorsals 

 supports very long and strong spines. 



The compressed piercing and trenchant form of tooth 

 which characterizes the existing varanian lizards was mani- 

 fested by the Megaloscmrus. The specimen which is most 

 illustrative of the dental peculiarities of this gigantic reptile 

 is a portion of the lower jaw with a few teeth, from the oolitic 

 slate of Stonesfield, Oxfordshire. The first character which 

 attracts attention in this fossil is the inequality in the height 

 of the outer and inner alveolar walls ; a similar inequality 

 characterizes the jaws of almost all the existing lizards. But 

 in these the oblique groove, so bounded, to which the bases of 

 the developed teeth are anchylosed, is much more shallow, and 

 is relatively wider ; and the teeth in all the stages of growth 

 are completely exposed when the gum has been removed. 



In the Megaloscmrus the greater relative development of 

 the inner alveolar wall, as compared with the dentigerous part 

 of the jaw in existing Saurians, deepens the dental groove, and 

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

 concealing more or less completely the germs of their succes- 

 sors. 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. 



