292 BRITISH FOSSIL REPTILES 



arch n; the base of this arch equals about two thirds of the antero-posterior extent of 

 the centrum to vvliich it is attached, a Uttle nearer the anterior than tlie posterior end. 

 The arch sends forwards a pair of long zygapophyses, z, whose articular surfaces look 

 inwards and a little upwards ; a low ridge traverses two thirds of the summit of the 

 arch, fig. 2, from the hinder third of which springs the neural spine, n s, which slightly 

 gains in antero-posterior extent as it rises : but its summit is broken away : the pos- 

 terior zygapophyses y, fig. 1 , project from the back part of the base of the spine : their 

 articular surfaces look outwards and a little downwards. 



The figure 2, of the vertebra, viewed from above, shows the form and extent of the 

 summit of the neural arch, which is rarely preserved in fossil vertebrae. Fig. 3 shows 

 the anterior, and fig. 4 the posterior, surface of the vertebra ; the articular ends of the 

 centrum are very shghtly and irregularly concave, with the margin thickened and 

 rounded ofi". The under surface of the centrum, fig. 5, is characterised by a median 

 groove or channel between two parallel ridges which extend from the anterior h to the 

 posterior A' htemapophysial surfaces. Of these the posterior one is the largest. 



The neural canal, figs. 3 and 4, n, is contracted ; its area is a full transverse oval. 



With respect to the terminal caudal vertebrae in which diapophyses and ha^mapo- 

 physes have ceased to be developed, no specimen of Iguanodon has yet been discovered in 

 which any such vertebrae have been so associated with the rest of the skeleton as to 

 enable the conscientious observer to determine their character as unequivocally 

 belonging to the Iguanodon. Two vertebras, from the Wealden, near Battle, in the 

 Museum of the Royal College of Surgeons, in which the diapophysis has subsided to a 

 longitudinal ridge, crossing the upper third of the centrum in the smaller specimen, 

 have been described in my 'Catalogue of the Fossil Remains' in that Museum, 4to, 

 vol. ii, p. 25, as probably belonging to the Iguanodon ; for it is most probable that the 

 typical form of the body of the Iguanodon's vertebrae is modified or lost in such terminal 

 and rudimental vertebrae ; but as these are, in every case, the least characteristic 

 bones of an extinct animal, their loss is of the least consequence, and any positive affir- 

 mation regarding them, on imperfect evidence, becomes the more gratuitous. 



Tympanic Bone of the Iguanodon. PI. 15. Nat. size. 



Of the bones of the head of the Iguanodon, the characteristic one above named, a 

 fragment of the upper jaw, and a larger proportion of the under jaw have been bi'ought 

 to light: the portions of the jaws, at least, are demonstrably those of the present 

 species of herbivorous reptile, by the teeth which they contain : the great Cetiosaur and 

 Streptospondyl may possibly have afforded the specimen figured in PI. 15, which was 



