ROOM III. LOWER JAW OF THE IGUANODON. 247 



the germ is at the bottom of the fang of the tooth in place ; 

 in others, the successional dental excavation is on the base 

 of the enamelled crown ; for in the Iguanodon the old teeth 

 were retained till nearly the entire coronal portion was worn 

 away, and the crown of the tooth, from the abrasion by use 

 above, and the removal of the fang by absorption below, was 

 often reduced to a mere disk, before it was finally shed. l 



As the surface of the crown, when abraded by mastication, 

 possesses two distinct facets, it is obvious that the arrange- 

 ment of the lower teeth in relation to the upper was inter- 

 mediate, or subalternate, as is the case in the ruminants. 



The external aspect of the specimen 2 (Lign 54) presents in 

 its transverse diameter a gentle convexity, traversed by a 

 slightly elevated longitudinal ridge, parallel with, and im- 

 mediately beneath, the row of vascular foramina commonly 

 met with in this part of the lower jaw in reptiles ; and towards 

 the posterior extremity, the side of the bone is somewhat com- 



LIGX. 54. EXTERNAL VIEW OF THE LOWER JAW OF THE IGUASODOX. 

 (} not. tize.) 



pressed below the longitudinal eminence; agreeing in this 

 respect with the portion of a lower jaw of a much smaller 

 reptile, to be described hereafter. 3 The upper margin of the 

 bone is formed by the outer alveolar parapet, which is deeply 

 scalloped or crenated by the terminations of the sockets of 

 the teeth ; the angular eminences indicate the intra-alveolar 



1 " Philos. Trans." 1848, p. 188. 



2 Figured in " Philos. Trans." 1848, PI. XVI. XVII. 



3 Described in " Philos. Trans." 1841 (PI. V. figs. 1, 8, 9), as part of 

 the jaw of a young Iguanodon, but which belongs to a distinct genus 

 the Regnosaurus. 



