WEALDEN FORMATIONS. 21 



The mandible of the Iguanodon from the Wealden of Tilgate, Sussex, figured 

 by Mantell in the 'Philosophical Transactions' for 1848, PI. xvii, seems to 

 have had at least twenty alveoli in a longitudinal extent of fourteen inches. The 

 back part of the series is too much mutilated for precisely showing the divisions 

 of the sockets ; but the number, eighteen, which I originally estimated, from the 

 figures of the fossil in the ' Philosophical Transactions,' is clearly below the 

 number which may be estimated in the alveolar tract of the original specimen 

 now in the British Museum. 



From the foregoing facts, therefore, it may be concluded that the Iguanodon, 

 in the progress of growth, from the period at which the dentigerous part of each 

 ramus of the mandible is four inches in length to that in which the same part is 

 fourteen inches in length, acquires four or five additional teeth in each series, which, 

 from the rapidly decreasing depth of the three or four hindmost alveoli, I infer 

 to be developed, like the true molars of mammals, in new and distinct alveoli 

 behind those in place. 



My obliging correspondent, Mr. Fox, who had been struck with the inferiority 

 of number of the alveoli in his small specimen, compared with the indication 

 of them in Mantell's plate of the larger jaw from Tilgate, supposed that it might 

 indicate a distinction of species ; but the whole evidence of the Iguanodon's 

 mandibular structure, including the intermediate-sized specimen obtained by 

 Mr. Holmes from Stammerham, appears to me to show only difference of age, 

 and to bring to light a new and important characteristic of the dentition of the 

 large extinct Herbivorous Reptile. 



