26 FOSSIL REPTILIA OF THE 



the lower half convex, the converging sides here terminating in a sharp ridge, which 

 is concave lengthwise (ib., fig. 6). 



In Streptospondi/lus major the centrum loses in length and gains in height ; the 

 neural arch at the same time augmenting in height as the vertebrae recede from the 

 neck. In the dorsal vertebra here described, there appears another Streptospondylian 

 character, pointed out by Cuvier in the Harfleur gavial-like species, the support, viz., 

 of the transverse process by ridges, converging to its base. The anterior ridge (ib., 

 fig. 2, a) ascends almost vertically in front of the surface (p, fig. 2) for the head of the 

 rib, the posterior ridge (e) , forming the outer and back part of the neural arch, ascends 

 obliquely forward to meet the first ridge beneath the diapophysis (rf). 



In the Iguanodon the first ridge {a, fig. 4) is hardly represented ; the second (e) 

 is well developed, but is nearly vertical. The chief difference, however, which the 

 vertebrae here compared of Iguanodon and Streptospondi/lus present, is seen in the 

 structure of the neural arch behind the posterior ridge. 



In Streptospondt/lus major the surface of the neural arch is continued from the 

 posterior ridge inwards and a little backwards, almost flat, to the thick, rounded border 

 of the posterior aperture of the neural canal, expanding with a slight concavity to the 

 base of the posterior zygapophysis. In Iguanodon the corresponding part of the neural 

 arch, viz., behind the posterior ridge (fig. 5, e), is excavated by a large and deep cavity. 



The neural canal in Iguanodon (fig. 5, n) is relatively smaller than in Streptospon- 

 di/lus, especially narrower, its area presenting the form of a vertical ellipse, whilst 

 in Streptospondylus it is a wide transverse ellipse (fig. 1, n). In Iguanodon a ridge 

 formed, as it were, by the lateral compression of the back part of the neural arch 

 between the two large hollows behind the buttresses of the diapophyses, rises vertically 

 to the median approximate extremities of the posterior zygapophyses (fig. 5, z'). A 

 broad, vertically convex surface, holds the place of the above ridge in Streptospondylus. 

 The forepart of the neural spine is thicker in Streptospondylus than in Iguanodon, and 

 there is a deeper and more circumscribed cavity on each side of that part of the spine 

 on the roof of the neural arch. The side walls of that arch are much thicker in 

 Streptospondylus, especially anteriorly, and the arch is shorter in proportion to the 

 centrum than in Iguanodon. With all these differences between answerable dorsal 

 vertebrae of Iguanodon and Streptospondylus, there remains the capital one of the front 

 ball and hind cup in the latter, where the corresponding surfaces are flat or very 

 slightly depressed in the Iguanodon. 



The determination of the true nature of the convexo-concave vertebrae of the 

 Wealden, and of the afiinities of the reptile to which they belonged, besides extending 

 our knowledge of tiie gigantic oviparous animals ot that epoch, removes one of the 

 chief difficulties attending the determination of the true vertebral characters of the 

 Iguanodon. For, if gigantic vertebrae, agreeing in the important character of their 

 articular surfaces with the existing Iguanas, had actually been discovered, though of 



