ROOM III. LOWER JAW OF THE IGUANA. 233 



attached by the external surface of the fangs to the inner 

 Bide of the alveolar process (as is shown in Lign. 49) ; and 

 as there is no mesial parapet of bone, the fangs of the teeth 

 are covered only by the soft parts (Jig. 1, Lign. 49). The 

 successional germs do not, as in the Crocodile, spring up in 

 the cavities of the mature teeth, and rise through them, but 

 are developed near the inner part of the base, and by their 

 upward growth occasion the absorption of the fang of the 

 old tooth, which is ultimately displaced and shed, from the 

 destruction of its adhesion to the alveolar parapet. In/$r. 3, 

 Lign. 49, the position of a germ at the base of the fang is 

 represented. 



The teeth of the Iguana closely resemble in form, but not 

 in structure, the perfect young upper teeth of the Iguanodon ; 

 they are very small, scarcely exceeding in size those of the 

 mouse. (In jigs. 1, 2, Lign. 49, the teeth are figured of the 

 natural size.) In the Iguana the crown of the tooth never 

 presents a worn or even surface ; it is broken or chipped off 

 by use, but not ground smooth as are the teeth of her- 

 bivorous mammalia. The reason is obvious: no existing 

 reptiles are furnished with cheeks or moveable coverings to 

 their jaws; they cannot perform mastication, but swallow 

 their food whole. 1 



LOWER JAW OF THB IGUANA. Lign. 49. Before I quit 

 this subject, and enter upon the examination of the teeth of 

 the Iguanodon, it will be convenient to explain the structure 

 of the lower jaw in reptiles ; and I select that of the Iguana, 

 as it will not only serve to illustrate the osteology of that 

 part of the skeleton, but also tend to elucidate the nature of 

 the highly interesting fossils that will hereafter engage our 

 attention. 



In mammalia the lower jaw is composed of one bone on 

 each side, and in many genera these pieces become united 

 in front, and blended into a single bone in the adult animal; 

 but in reptiles this element always consists of six distinct 

 pieces on each side, and these several parts are variously 



1 There are some of the large Monitors which can give a semi- 

 rotatory motion to the back teeth ; but in no living species of reptile 

 is the trituration of the food effected as in the mammalia and the 

 Iguanodon. 



