304 BRITISH FOSSIL REPTILES. 



of the lower jaw, in itself significant of some correlative modification of the upper jaw. 

 So far as the valuable fragment in question illustrates the nature of that modification, 

 we discern in it an approximation to the mammalian type of the superior maxillary- 

 bone, subservient probably to a greater development of the homologue of the masseter 

 muscle than is found in any recent reptile. 



As the lower jaw of the Iguanodon does not contain more than 18 teeth in each 

 ramus, it may be concluded that the portion of the upper jaw above described, 

 supported at least one half of the dental series of the left side. The total length of 

 that series in the skull to which such portion of jaw belonged must have been about 

 16 inches. The length of the alveolar ti-act, in the largest example of a ramus of the 

 lower jaw yet discovered, PI. 18, fig. 1, is 13 inches. 



In a cranium of the Iguana tuberculata, which measures 2\ inches in length, 

 the dental series occupies four sevenths of that length : according to the same 

 proportions, therefore, the cranium of the Iguanodon, affording the above fragment of 

 the upper jaw, would be 2 feet 4 inches in length. If the lower jaw of the Iguanodon 

 exceeded the length of the cranium in the same proportion as in the Iguana, 2 feet 

 8 inches may be assigned to the total length of the skull of the Iguanodon, according 

 to the evidences as yet obtained. But the unbiassed will feel that the rest of the 

 structure of the Iguanodon, and especially of its teeth arid vertebral column, differs in 

 too great and important a degree from that of the Iguana to allow much confidence to 

 be attached to the conclusions formed or suggested as to the Iguanodon, according to 

 the osteology of the recent lizard, after which it has been called. 



Teeth of the Iguana. PI. 23. (Figs. 1—5, after Mantell, ' Phil. Trans.') 



Respecting these characteristic parts of the great extinct Reptile, little need be 

 added to the observations recorded in my ' Odontography,' in the ' Section on 

 the Fossil Reptilia of the Cretaceous Formations,' pp. 269 — 272, and the excellent 

 descriptions by Drs. Mantell and Melville, in the ' Philosophical Transactions,' for 

 1848. 



Fig. 1, is a fully formed and moderately worn tooth of the upper jaw, showing the 

 outer side ; a, is the submedian primary longitudinal ridge, b, h, the accessory ridges, 



c, c, the lamello-serrated margins of the crown, of which the anterior is the longest ; 



d, d, the compressed sub quadrate fang. Fig. I «, gives a view of the fore part of the 

 same tooth, showing the varying proportions of the two diameters of the crown and fang. 

 Fig. 1 b, gives the form of the grinding surface of the crown ; a, is the primary ridge 

 on the enamelled side ; b and c, the two facets produced by the attrition of two opposed 

 teeth on the lower jaw. 



Fig. 2. The outer side of an old tooth from the left upper maxillary bone, of 



