ROOM III. LOWER JAW OF THE IGUANODON. 245 



measure broken away ; within this process the germs of the 

 successional teeth were developed. 



The mode of implantation of the teeth appears to have been 

 intermediate between the pleurodont and thecodont types, 

 for the teeth were not anchylosed to the alveolar wall as in 

 the Iguanas ; yet as the ridges that separate the dental 

 sockets are smooth and rounded, it may be inferred that 

 these were not rendered complete alveoli by transverse plates 

 extending from the outer to the inner parapet, as is the case 

 in the Megalosaurus. 1 



The dental sockets diminish in size, but somewhat irregu- 

 larly, from the posterior to the anterior termination of the 

 alveolar process ; and the latter suffers a corresponding dimi- 

 nution in breadth, and terminates suddenly at the distance of 

 five inches from the front. At this point the upper margin 

 becomes attenuated and contracted in a vertical direction, and 

 descending with a gentle curve, expands horizontally and 

 niesially to unite at the symphysial suture with the opposite 

 ramus ; the anterior part of the jaw being edentulous. 



From the fortunate preservation of two successional teeth 

 in their original position, the mode of dental development in 

 the Iguanodon is clearly demonstrated. As in existing saurians, 

 the germ of the coronal portion of the tooth was first secreted, 

 and the entire crown completed before the formation of the 

 shank or fang commenced. The formative pulp was situated 

 in a distinct depression or cavity, on the inner face of the 

 root of the tooth it was destined to supplant : this is obvious 

 by the position of the teeth above described ; and also by the 

 remains of a third germ, which is observable towards the 

 posterior part. 2 



Although the peculiar characters of the molars of the 

 Iguanodon have already been described somewhat in detail, 

 and the present fossil confirms in every essential particular 

 the inferences suggested by the detached teeth, yet several 

 new and important points relating to the development and 

 functions of the dental organs, are elucidated by the new 



1 See Dr. Buckland's " Bridgewater Essay," PL 23. 



2 A reference to the lithograph in the " Philos, Trans." PL XVI. 

 representing the specimen of the natural size, is necessary for the full 

 illustration of this description. 



