WEALDEN FORMATIONS. 53 



Now the largest vertebra of an Iguanodon which has yet been obtained does not, as 

 has been before stated, exceed 4| inches in length ; the most common size being 4 

 inches. The intervertebral substance is shown, by the naturally juxtaposed series of 

 dorsal vertebrae in the Maidstone Iguanodon, to be not more than one third of an inch 

 in thickness. All the accurately determined vertebrae of the Iguanodon manifest the 

 same constancy of their antero-posterior diameter which prevails in Saurians generally; 

 the discovery of the true character of the supposed Lacertian vertebrae, six inches in 

 length, removes the only remaining doubt that could have attached itself to this important 

 element in the present calculation.* The anterior cervical vertebrae of the Iguanodon, 

 when discovered, if they prove to differ in length from the known dorsal and caudal 

 vertebrae, will be, in all probability, somewhat shorter, as they are in the Hylaeosaur 

 and in all known Crocodiles and Lizards. It remains, therefore, to determine the most 

 probable number of the vertebrae of the Iguanodon, in order to apply their length 

 individually to the estimate of the length of the entire body. The structure of the 

 vertebrae and the ribs, and especially the variation in both, structure and size which the 

 ribs of the Iguanodon, already obtained, demonstrate to have prevailed in the costal 

 series, render it much more probable that the number of the costal vertebrae would 

 resemble that of the Crocodiles than that of the Scincus or other Lizards with unusually 

 numerous dorsal vertebrae, and which possess ribs of a simple and uniform structure, 

 and of nearly equal size. The most probable number of vertebrae of the trunk, from 

 the atlas to the last lumbar inclusive, calculated from Crocodilian analogies, would be 

 24 vertebrae. This is the number indicated by the instructive portion of the skeleton 

 of the young Iguanodon figured in Tab. I, and for the first time described in the present 

 Monograph : it is also the number possessed by the Iguana. 



Twenty-four vertebrae, estimated with their intervertebral spaces at 5 inches each, 

 give 10 feet; if to this we add the length of the sacrum, viz., 17 inches, then that of 

 the trunk of the Iguanodon would be 1 1 feet 5 inches ; which is about equal to that of 

 the Megatherium. If there be any part of the skeleton of the Iguana which may with 

 greater probability than the rest be supposed to have the proportions of the corres- 

 ponding part of the Iguanodon, it is the lower jaw, by virtue of the analogy of the 

 teeth and the substances they are adapted to prepare for digestion. Now the lower 

 jaw gives the length of the head in the Iguana, and this equals the length of six dorsal 

 vertebrae, so that as 5 inches rather exceeds the length of the largest Iguanodon's 

 vertebra yet obtained, with the intervertebral space superadded, on this calculation 

 the length of the head of the largest Iguanodon must have been about 2 feet 6 inches, 

 and this is nearly the length of the head, as estimated on the data afforded by 

 the portions of lower jaw described at pp. 20 — 30. In the description of 

 the caudal vertebrae it has been shown that the Iguanodon could as little have 



* Reports of Brit. Association, 1841, p. 92. 



