302 BRITISH FOSSIL REPTILES. 



a thin layer of dentine has been formed beneath the enamel; the mineral 

 matter now occupies the place of the original vascular pulp of the dental 

 matrix. The flattened side of the crown of this tooth is turned towards the outer 

 alveolar wall, the convex surface looks inwards and downwards ; in the lower jaw the 

 teeth, PI. 16, fig. 2, /, /, have the reverse direction, as stated in Dr. Mantell's Memoir 

 on the lower jaw, from Tilgate.* Next, behind the young tooth, 2, is the recess from 

 which an old tooth has been expelled ; and behind the recess is a fully formed crown 

 of a tooth, 3, with the beginning of the fang, which tooth had come into use, but its 

 grinding surface has been worn down by the rolling of the fragment after fossilization 

 and extrication of the specimen from the matrix ; a narrow recess follows this tooth, and 

 then comes the fang and base of the crown of an old tooth, 4, partly undermined, and 

 about to be pushed out by the crown of a successor, r> ; next follows an empty recess ; 

 then the base of apparently a fully developed tooth, 6, the projecting crown of which 

 has been broken away ; close behind this tooth is the base of a narrower and smaller 

 tooth, 7, followed by the recess for a similar sized tooth, which terminates the series. 



We thus see that, as in the lower jaw of the Iguanodon and in the upper jaw of the 

 Iguana and Tejus, the teeth decrease in size at the hinder end of the series ; and that 

 this end of the series in the Iguanodon inclines outwards, as does the same end of the 

 alveolar series in the lower jaw, to which it was opposed. 



As a similar portion of bone, recognised by Dr. Mantell as a " fragment of the 

 upper jaw of an Iguanodon," when first discovered in 1838, in a quarry near Cuckfield, 

 has been referred to the opposite end of the jaw, in the Memoir in which it is figured, 

 'Philosophical Transactions,' 1848, PI. XIX, pp. 190, 191, with an appeal to the 

 osteology of the recent Iguana, as confirmatory of that determination, I may be excused 

 for concluding by a summary of the facts which seem to me to determine rightly the 

 nature and relative position to the rest of the skull of the present very interesting part 

 of the fossihzed skeleton of the Iguanodon. The size of the teeth forbids the suppo- 

 sition that the fragment in question can have formed part of a pterygoid or palatine 

 bone, — such a dentigerous bone, viz., as is shown in the skull of the Mosasaurus and, 

 amongst existing Saurians, in the Iguana : l)oth the shape of the pterygoid and the 

 relative size of the teeth discourage the idea that the present fragment can be part of 

 the homologous bone : it would be contrary to all known analogies to refer it to the 

 palatine bone ; and there remains, tliereforc, only the superior maxillary bone with 

 which to compare it. Of this bone the specimen is evidently that part or extremity 

 containing a natural termination of the alveolar groove ; this is shown by the suddenly 

 diminished size of the teeth and alveoli, and by the portion of bone, p, fig. 2, which is 

 continued beyond the last alveolus. 



The question next arises : — Does the fragment include the anterior or posterior end 

 of the alveolar groove ? In answer to this I may first remark, that the outer and inner 



* PLilos. Trans., 1818, p. 187. 



