318 GEOLOGY OF THE ISLE OF WIGHT. 



with, three or four longitudinal ridges down the 

 front, and a broad denticulated margin on each 

 side.* The resemblance between the unworn teeth 

 of the Iguanodon and those of the Iguana, a 

 herbivorous lizard of the West Indies, suggested 

 the name of this colossal extinct reptile ; but the 

 teeth of the Iguana are very small, not exceeding 

 in size those of the mouse. 



Sections of the teeth are beautiful objects under 

 the microscope, and show that the structure of 

 the dentine, or tooth-ivory, is coarser than in 

 other reptiles, and resembles that which charac- 

 terises the teeth of the Sloth tribe. The crown 

 of the tooth is incrusted with an external coating 

 of enamel, which is thick in front, and thin on the 

 inner surface. In consequence of this disposition, 

 and of the dentine being hardest in the anterior 

 part, the grinding surface of the tooth is always 

 worn away obliquely, and a sharp cutting front 

 edge of enamel maintained in every stage. if 



Sacrum of the Iguanodon. — A very remarkable 

 peculiarity in the anatomical structure of the 

 Iguanodon, and of the Hyheosaurus and Megalo- 

 saurus, consists in the sacrum being composed of five 

 vertebrae an chylosed, or fixed together, into a solid 



* Medals of Creation, \>. 741. 



t Wonders of Geology, veil. i. p. 390. 



