22 FOSSIL REPTILIA OF THE 



Eighteen alveolar fossae for the lodgement of the contracted sub-cylindrical bases of 

 the teeth are exhibited in Mr. Holmes's specimen ; but all the teeth that were fully 

 developed and had occupied those semi-cylindrical depressions have been lost. 

 Greater or less portions of the protruding summits of six successional teeth are seen 

 below the alveolar grooves of the old teeth, and of so much larger size as indicates a 

 more rapid growth of the young Tguanodon, than in modern reptiles. In the different 

 proportions in which the young teeth are developed, may be discerned an illustration 

 of the same law of preservation of an adequate proportion of an ever changing series 

 of masticatory organs, which is illustrated by the condition of the dental series in 

 many modern reptiles and fishes. The teeth marked k, k, k, for example, of which the 

 summits of the crown have but just begun to be calcified, alternate with those marked 

 /, I, I, fig. 2, Tab. XI, in which the crowns are more advanced. One may see by the size 

 of these teeth that they are destined for work in a larger jaw than that of the young 

 Iguanodon in which they are cradled ; one may likewise discern the unfitness of the 

 actual alveolar grooves for the reception and retention of the large successional teeth, 

 and thence rightly infer that the bone grows and goes with the growth and disap- 

 pearance of the teeth themselves ; the alveoli of the shed teeth being progressively 

 absorbed as the osseous bed of the new teeth rises along with them. The same 

 concomitant growth of the jaw-bone and the teeth has long been recognised in the 

 mammalian class, and is strikingly exemplified in the elephant, in which the large 

 complex molars succeed each other from behind forwards. 



The surface of the jaw below the alveolar groove is smooth, but is traversed by a 

 deeper and narrower groove continued from the entry of the mandibular canal, *» 

 forwards just above, and nearly parallel with, the lower border of the ramus, becoming 

 shallower and descending to that border as the groove, d, approaches the symphysis, « ; 

 the major part of this groove was probably covered by the splenial element, (opercular 

 of Cuvier), in the entire ramus of the Iguanodon's jaw. Above the groove the inner 

 surface of the dentary is slightly convex at its posterior half, and slightly concave at 

 the anterior half. The edentulous, narrow, sloping margin of the jaw, b, e, has a slightly 

 tumid roughness along its inner side, as if for the firm attachment of a callous covering 

 in the recent animal. The actual symphysis of the jaw is about two thirds of an inch 

 in extent, and a quarter of an inch in greatest depth, almost horizontal in position, but 

 bent, with the concavity looking upwards ; the inferior and anterior angle of the jaw, 

 d, projects a little way beyond the fore part of the symphysis, and the back part of the 

 symphysis is impressed with a longitudinal groove, fig. 2, s, parallel with, but above, 

 the anterior end of the mandibular groove, d. 



In the small extent of the mandibular symphysis the Iguanodon resembles the 

 Lacertilia, and differs from the Crocodilia, even from the true crocodiles and alligators 

 in which the symphysis is much less than in the gavials ; but the position of the 

 symphysis at the lower end of the anterior termination of the ramus, and the sloping 



