WEALDEN FORMATIONS. 29 



sides of the fragment are determined by the relative depth of the walls of the alveolar 

 groove, and by the relative position of the new and old teeth. In no pleurodont lizard 

 is the deeper wall the innermost ; and in no lizard or crocodile does the germ of a 

 successional tooth appear on the outside of the base of the one it is about to succeed. 

 The philosophy of Zootomy compels one to be guided by so great a number of observed 

 instances, as is implied by the above generalized statement, as by a rule; and we know 

 that the lower jaw of the Iguanodon conforms to that rule, by direct observation. In 

 the upper jaw of the Iguanodon the successional tooth-germ is not situated directly on 

 the inner side, but is also behind the tooth about to be displaced, at least in most of 

 the specimens in the present fragment. 



The extremity of the alveolar series, therefore, exhibited in the present fragment, 

 must be either the fore end of the right maxillary bone or the back end of the left 

 maxillary bone. The expansion and bifurcation of the bone, as it approaches towards 

 the end of the alveolar series, are opposed to every analogy presented by the fore part 

 of the maxillary in the Lacertian and Crocodilian reptiles. The foramina, grooves, 

 and sutural surfaces become utterly unintelligible in this supposition ; which is opposed, 

 moreover, by the direction of the nervo-vascular outlets on the outer side of the bone, 

 and by the curvature of the extremity of the alveolar series, as compared with the 

 anterior extremity of that series in the lower jaw. In favour of the conclusion that 

 the fragment in question is from the back part of the upper jaw, the expansion of the 

 bone as it recedes from the triedral fractured end, a, a', the direction of the nervo- 

 vascular outlets, g, g, the altered direction of the alveolar groove, inclining, e. g., 

 outwards to be adapted to the hinder curve of the alveolar groove of the lower jaw, 

 and the diminished proportions of the teeth at its obvious termination, all concur. 

 And I may add that, supposing the Iguanodon, like the Iguana, to have had the dental 

 series of the upper jaw prolonged forwards upon a premaxillary bone, the alveolar 

 series of the maxillary would have been continued nearer to the end of the bone, and 

 would have terminated more abruptly than it does in the present fragment. 



Thus conducted to the conclusion that we have in the fragment in question the 

 hinder part of the left superior maxillary bone, we have evidence that the Iguanodon 

 differed (as, indeed, from the important differences in other parts of the skeleton might 

 have been expected) from the Iguana and the Crocodiles, in having the alveolar end 

 of the upper jaw produced backwards, beyond that outstanding backwardly inclined 

 process, which gave attachment to the malar bone, such backwardly produced dentary 

 end of the bone corresponding with that end, in the existing reptiles above cited, which 

 articulates with the ectopterygoid (" os transverse" of Cuvier). 



The dental series, thus brought more beneath the cranial part of the skull, would 

 be more favorably placed for the operations of the masticatory muscles inserted into 

 the lower jaw, and the backward prolongation of the dentary element, where it is 

 developed into a coronoid process, is a departure from the ordinary reptilian structure 



