On BRITISH FOSSIL REPTILES. 143 



pared by Dr. Mantell *, would yield a result of upwards of 200 feet for the total 

 length of the Iguanodon, since the Horsham phalanx exceeds the size of the 

 largest of the recent Iguana's phalanges by 40 times ! 



But the same reasons which have been assigned for calculating the bulk of 

 the Megalosaurus on the basis of the vertebrae, apply with equal force to the 

 Iguanodon. 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 Iguano- 

 don, 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 im- 

 portant element in the present calculation-}-. The 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 re- 

 mains, therefore, to discover 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 Igua- 

 nodon, 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 ; which 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 11 feet 5 inches; which 

 exceeds 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 corresponding 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,onthis calculation the lensjth 

 of the head of the \a,rgest Igutmodon must have been 2 feet 6 inches. In the 

 description of the caudal vertebrae it has been shown that theIguanodo7i could 

 as little have resembled the Iguana in the length of its tail;]:, as in the anato- 

 mical characters of any of the constituent vertebra of that part : the chano-es 

 which the series of six caudal vertebrae present in the length and form of the 

 spinous processes, and in the place of origin of the transverse processes, indi- 

 cate the tail to have been shorter in the Iguanodon than in the Crocodile. 

 Assuming, however, that the number of caudal vertebrae of the Iguanodon 

 equalled that in the Crocodile, and alloAving to each vertebra with its inter- 



* Mantell, Geology of the South-east of England, -p. 314. 

 t See p. 92 of the present Report. 



X See also the judicious remarks hy Dr. Buckland to the same effect, Bridgewater Treatise, 

 p. 244. 



