ON BRITISH FOSSIL REPTILES. 125 



to the Iguanodon, indicative of a peculiarity of the joint of the lower jaw as 

 remarkable as the structure of the teeth, and correlated to their masticatory 

 uses ? " Two lateral processes, or alcE, pass off obliquely, and are small in 

 proportion to the size of the column; on placing these bones beside the os 

 tympani of an Iguana, we at once perceive that the relative proportions of 

 these parts are reversed ; for in the recent animal the pillar is small and the 

 lateral processes large. From the great size of the body of the fossil, and the 

 extreme thinness of its walls, the tympanic cellulcB must have been of consi- 

 derable magnitude, and have constituted a large portion of the auditory cavi- 

 ties. PI. ii. fig. 1., (fig. 5 is meant,) accurately represents the most perfect 

 specimen in my cabinet ; it is 6 inches high, and 5| inches wide at the 

 longest diameter of the extremity of the body. It exceeds in magnitude the 

 corresponding bone of the Mosasaurus, and is fourteen times as large as the 

 same bone in an Iguana 4 feet long." Loc. cit., p. 306. 



Vertebral Column. — The vertebrae of the Iguanodon have their bodies ter- 

 minated by flat or slightly concave articular surfaces*, and their sides flat or 

 slightly convex vertically, moderately concave lengthwise or in the axis of 

 the vertebra ; the sides converge more or less towards the under surface, 

 and the body accordingly presents more or less the form of a wedge, with its 

 edge obtuse or flattened in the dorsal vertebras, but slightly concave, and with 

 its anterior and posterior angles truncated in the caudal vertebrae f. The con- 

 tour of the terminal surfaces is nearly circular, with the vertical slightly exceed- 

 ing the transverse diameter. The neural arch of the dorsal vertebrae presents 

 the complicated exterior, the great height and superior expansion, which cha- 

 racterize these vertebrae in other Dinosaurs : the base of each neurapophysis 

 equals^ or nearly equals, the antero-posterior extent of the centrum, but im- 

 mediately contracts in this direction from the posterior margin, which then 

 curves backwards as it inclines towards the opposite neurapophysis, and the 

 conjoined laminae are developed beyond the posterior end of the centrum 

 to an extent varying in the different regions of the spine. In the dorsal ver- 

 tebras the bases of the neurapophyses are developed transversely inwards, so 

 as to meet and join each other below the spinal canal : the haemapophyses pre- 

 sent an analogous structure through a great part of the tail, the bases of each 

 pair, as well as the apices, being united together, and the chevron bones, thus 

 formed, are perforated instead of being notched for the passage of the great 

 blood-vessels. The neurapophyses are commonly anchylosed to the centrum, 

 with a persistent trace of the suture. The transverse processes are straight, 

 and of great length in the vertebrae from the middle of the trunk, indicating 

 there a considerable expanse of the abdominal cavity, adapted for the lodge- 

 ment of the capacious viscera of a herbivorous feeder. The spinous processes 

 rise to a considerable height in the dorsal, as well as in the anterior caudal 

 vertebrae. The exterior surface of the vertebrae is impressed with fine stri- 

 ations, which are mostly longitudinal in the centrum ; so that fragments may 

 thus be distinguished from the characteristically smooth and polished vertebrae 

 of the 3Iegalosaur%is. The antero-posterior diameter of the largest vertebrae 

 of the Iguanodon which I have yet seen is 4^ inches ; the most usual size is 

 4 inches. 



Having premised these general characters of the vertebrae of the Iguano- 

 don, there next remain to be considered the modifications by which tlaey are 



* The plano-coiicave vertebrse in tlie Mantellian Collection, British Museum, beloug to 

 the Cetiosaurus. 



t The large vertebr?e from the Wealden, with obscurely quadrangular or hexagonal bodies, 

 which are rather convex or flat on one side and concave on the other, belong to the Ceiio- 

 taurus. 



