246 IGUANODON. 



characterized by large irregularly shaped calcigerous cells, and the 

 interspaces are filled with irregular moss-like reticulations of tubes, 

 (ib. a) . Branches of the pulp-cavity are never continued in the form 

 of medullary canals into the substance of the dentine in the existing 

 Iguanse. 



The arrangement of the tubes, as seen in a transverse section at 

 the middle of the tooth is represented at PL 69, fig. 1 ; & is the 

 tubular dentine, c the enamel. The secondary curves and terminal 

 divisions of the calcigerous tubes are shown in fig. 3, but the small 

 oblique secondary branches are not accurately expressed in this 

 figure. 



The germs of the successional teeth are developed from the 

 mucous membrane covering the inner side of the base of those in 

 place. The apex of the dentated crown is first formed ; by its pres- 

 sure it excites absorption of the base of the fixed tooth and soon 

 undermines it, and then occupies the recess in the alveolar plate in 

 the interspace of the two adjoining fixed teeth. After the crown is 

 completed, the rest of the tooth forms a contracted and elongated fang, 

 which at first is hollow, then becomes consolidated by ossification of 

 the remaining pulp, and afterwards a second time excavated by the 

 pressure of a new tooth. 



IGUANODON. 



103. The value of a knowledge of the comparative anatomy of 

 the teeth, and especially of their external characters in the cold-blooded 

 classes of animals, has never, perhaps, been placed in so striking a 

 point of view as in the leading steps to the discovery of the present 

 most extraordinary and gigantic reptile ; these, therefore, I shall re- 

 count in the words of Dr. Mantell, to whom is due this splendid acces- 

 sion to the riches of Paleeontology. 



After noticing the ordinary organic remains which characterize 

 the sandstone of the Tilgate forest, and his discovery, in the summer 

 of 1822, of other teeth distinguished by novel and remarkable charac- 

 ters, the indefatigable explorer of the Wealden proceeds to state, 



" As these teeth were distinct from any that had previously come 

 under my notice, I felt anxious to submit them to the examination of 



