WEALDEN FORMATIONS. 45 



herbivorous Saurian that modification which is less incurved than the typical form in 

 the Iguanodon, and which exhibits that straighter and more conical form of phalanx, 

 Tab. XVII, figs. 1, 2, 3, (No. |& " Horn of the Iguanodon" Mantellian Collection 

 and Catalogue) described in p. 141 of my * Report on British Fossil Reptiles,'* and the 

 determination of which, as a phalanx, in that ' Report,' subsequent acquisitions of 

 similarly modified phalanges, e. g., figs. 4, 5, Tab. XVII, have served to confirm. 



As, however, the original opinion of the indefatigable explorer of the Wealden, to 

 whom we owe our chief knowledge of that formation in England, has continued to 

 prevail in the numerous geological and palaeontological works published since 1841, 

 it is incumbent on me to enter more into detail relative to the fossil on the nature of 

 which I found myself compelled to differ with its original discoverer. 



A certain resemblance in outward form, which the fossil teeth of the Iguanodon 

 present to those of the Iguana, has exercised, as I have already intimated, undue 

 influence in the prevalent ideas as to the affinities of the gigantic herbivorous reptile 

 of the Wealden to the small existing lizard, afterwhich.it has been named. The 

 Iguanodon, indeed, is generally supposed to have been characterised by a singular 

 structure, viz., a horn, like that which, in the existing order of Saurians, distinguishes 

 one of the species of Iguana, (Metopoceros, or Iguana cornuta). 



The following observations on the fossil which has given rise to that opinion, may 

 tend in some degree to modify, and I believe to rectify the received ideas as to the 

 nature and affinities of the Iguanodon. 



The bone to which I allude is that which Dr. Mantell has described as the "horn 

 of the Iguanodon" in the following words, which convey an accurate idea of its general 

 form and size. 



" We have," says Dr. Mantell, " to request the reader's attention to a very remark- 

 able appendage with which there is every reason to believe the Iguanodon was 

 provided. This is no less than a horn, equal in size, and not very different in form, to 

 the upper horn of the rhinoceros. This unique relic is represented of the natural size, 

 PI. XX, fig. 8.f It is externally of a dark brown colour, and while some parts of 

 its surface are smooth others are rugous and furrowed, as if by the passage of blood- 

 vessels. Its base is of an irregular form, and slightly concave. It possesses an 

 osseous structure, and appears to have no internal cavity. It is evident that it was 

 not united to the skull by a bony union, as are the horns of the mammalia." 



The only reason which I have, hitherto, been able to find adduced for the above 

 determination of the fossil described as " the horn of the Iguanodon," is, that a species 

 of Iguana has, on the middle of its forehead, an osseous conical horn or process 

 covered by a single scale."^ 



* ' Reports of the British Association,' 1841. 



f 'Illustrations of the Geology of Sussex,' 4to, 1827, p. 78, pi. xx, fig. 8. 



% Loc. cit., cited from ' Shaw's Zoology.' 



