246 PETRIFACTIONS AND THEIR TEACHINGS. CHAP. III. 



acquisition which Captain Brickenden has so generously 

 placed at my disposal, The second tooth, which occupies its 

 natural position in the alveolar space, consists of a perfect 

 crown, 1 1 inch in height, with the serrated margin as sharp 

 as when recent ; and this was the first evidence obtained as to 

 the mode in which the teeth were implanted. The flat 

 enamelled face of the tooth, characterized by its longitudinal 

 ridges, is placed mesially, or towards the inside of the mouth, 

 and parallel to, and within the inner alveolar wall j the 

 smooth convex part of the crown fills up a depression in the 

 outer parapet, in the interspace of two sockets of the mature 

 molars. This position is the reverse of that in which the 

 successional teeth in the Iguana are developed ; for in that 

 reptile the coronal germ occupies the same relative place as in 

 the mature state, the ridged face being outwards, and the 

 smooth side inwards, or towards the cavity of the mouth. 



As the crown of the tooth in the Iguanodon is not sym- 

 metrical, one lateral margin presenting a gentle curvature, 

 and the other forming a broad angle at the base of the ser- 

 rated border, the teeth belonging to one side of the lower jaw 

 may readily be distinguished from those of the other ; the 

 lateral marginal angle being always situated posteriorly. 

 Guided by this character, Dr. Melville and myself examined 

 the numerous teeth in the British Museum and in my own 

 collection, and were enabled to ascertain to which ramus or 

 side of the jaw any tooth belonged. Thus, for example, the 

 specimen represented, Lign. 50, which is a perfect successional 

 germ, the counterpart of that implanted in the jaw, consisting 

 of the crown before the formation of the fang, belongs to the 

 right side. 



The situation of the germ in relation to the tooth it was 

 destined to supplant, is invariably on the inside of the mouth ; 

 in the lower molars the excavation in the mature tooth occa- 

 sioned by the upward growth of the germ, is consequently on 

 the enamelled mesial or inner face, as is shown in my original 

 memoir : l in the upper tooth the germ was lodged in an 

 excavation on the smooth convex aspect. 2 



In some examples the cavity produced by the pressure of 



1 Philos. Trans." 1825, PL XIV. fig. 7 a. 



2 Philos. Trans." 1848, PL XVIII. fig. 2",/. 



