THE IGUANODON AND HYL^OSAURUS. 
291 
by the pinched up lower edge of the coalesced peduncles of the rnet-arthrapophyses, 
which are unfortunately detached. The spinal canal is nearly circular, and expands 
slightly in front, where it assumes a transversely oval outline. 
In a corresponding anterior dorsal vertebra. No. 2160 of the Mantellian Collection, 
Plate XXVIll. fig. 7, belonging to a younger and smaller individual, the posterior 
articular processes are present (g), and the perapophysial surface (/) is well-defined, 
but has in the Report on British Reptiles been regarded as the base of the transverse 
process, whilst the true origin {d) of that process is stated to be ‘ the rough external 
free border’ of the spinal platform, ‘probably fractured.’ A comparison of figs. 7 
and 8 will remove any doubt as to the accuracy of the interpretation here adopted. 
The wedge-shaped form of the centrum in the above-mentioned vertebrae cannot be 
regarded of higher value than as indicating their anterior position in the dorsal series ; 
in the Crocodile, the compression of the centrum and the development of an inferior 
carina ceases in the fifth dorsal, in which also the head of the rib is attached to a 
facet on the transverse process a little external to its base, while the tubercle is fixed to 
its extremity, as is the case in the vertebra, Plate XXIX. fig. 9 ; which from its close 
resemblance to those just described we have ventured to assign to the Iguanodon, 
notwithstanding those slight modifications which have induced Professor Owen to 
regard similar ones as belonging to the genus Cetiosaurus, but which we believe to 
be simply indicative of position in the same vertebral column, as we have wholly 
failed in detecting any such differential characters, after repeated examination, as 
would warrant us in considering this vertebra as specifically, and still less generi- 
cally, distinct. 
This vertebra (Plate XXIX. fig. 9) differs from those above described in the relative 
shortness and in the cylindrical form of the body, which is much constricted in the 
centre, so that the surfaces are deeply concave parallel to the axis, but convex in the 
opposite direction. Its length is 3 inches 6 lines ; the width of its anterior subcircular 
articular facet is 6 inches 1 line, inclusive of the thick rough everted edge, and its 
height 5 inches 4 lines. The posterior surface is transversely oval ; both surfaces are 
somewhat concave, but the hinder more distinctly so, especially in its upper half, whilst 
the corresponding part of the anterior aspect is raised into a faint mesial convexity ; 
the adjacent surfaces of contiguous vertebrae are thus coadapted. The spinal canal 
is 1 inch 1 line transversely where narrowest, but enlarges anteriorly. The neuro- 
pomal sutures are obliterated, but the direction of the superficial striae or rugosities 
indicate the great expansion of the bases of the neural laminae, which leave only a 
narrow tract widening behind the centrum to form the floor of the spinal canal. 
The neuropome rises from its base nearer the anterior than the posterior surface, and 
thus the intervertebral foramen is chiefly constituted by the posterior notch. Where 
most contracted the neural lamina measures 2 inches 6 lines in antero-posterior ex- 
tent, at its base it is 2 inches 10 lines; seven lines of the body are left exposed 
behind, and about three in front. But who will venture to base generic distinctions 
on such trivial characters as these ? The enormous spine rises from nearly the whole 
