294 
DR. MANTEL L ON THE OSTEOLOGY OF 
occurring in the Wealden, to those from the same deposits and localities belonging 
to other regions of the spinal column, all referable to the Iguanodon, excepting the 
few megalosaurian and crocodilian vertebrse, is such as long ago to have induced 
Dr. Mantell to regard them as characteristic of that Saurian ; and the occurrence 
of such vertebrse with those of the sacrum and other bones of the Iguanodon in 
Western Sussex, described by Cuvier, has already been commented on*-.' — 3rdly, 
as I shall presently show that the four large anterior caudal vertebrse in the Man- 
tellian Collection, also assigned by the author of the Report to the Cetiosaurus hrevis, 
cannot be transmuted into the vertebrse in question by any changes occurring 
in a consecutive series, there is left for that animal only some terminal caudal verte- 
brse ; while to complete the tail of the Iguanodon just those are wanting; 4thly, but 
independently of the evidence furnished by the Maidstone specimen, we have seen 
examples which point out the series of changes by which these angular vertebrse are 
produced from those of the middle caudal region. These changes, again, are not 
greater than those that take place in the tail of the Hylseosaurus'l- and other extinct 
reptiles, as well as in that of many mammalia. * 
Let us look for a moment at the vertebrse of the tail of the Mosasaurus as con- 
trasted with those of other regions of the spinal column in that reptile, and we shall 
then be prepared to admit far greater modifications than are here assumed. Could 
we a priori correctly restore the vertebral column of any animal from scattered 
fragments, belonging to different individuals, without making allowance for the 
changes occurring in the series of segments composing that column ? 
In the form of the terminal caudal vertebrse we may expect to find a very great 
similarity even in remote genera, and hence it is unsafe to base a generic character 
on their peculiarities. The genus Cetiosaurus (restricted to the species mediiis and 
longus from the oolite) is founded chiefly on such trivial distinctions, and we may 
refer to it any caudal vertebra of considerable dimensions with plano-concave or 
biconcave facets not referable to other known and perfectly determined genera, such 
as the Ichthyosaurus and Plesiosaurus, of which we have fortunately nearly perfect 
skeletons, and hence cannot be led astray in the labyrinth of fragments from which 
we are compelled, in most instances, to construct the lost denizens of tlie former 
lands and seas of our globe. 
In the caudal vertebrae of the Iguanodon, the body is wedge-shaped; the sides, 
which are faintly concave lengthwise and flat, or but slightly convex vertically, con- 
verge towards each other below; in the three or four most anterior, they present a 
concavity beneath the base of the short caudal rib, which is wedged between the cen- 
trum and the root of the neural lamina ; in a very instructive example in Dr. Mantell’s 
Collection, the pleural element has dropped out from one side, leaving a deep cavity 
now filled by matrix;}:. The caudal ribs disappear towards the middle of the tail, after 
which the bodies of the vertebrse have a subhexagonal form, Plate XXX. figs. 12, 13 ; 
* Ante, p. 277. t See Plate XXXII. % Philosophical Transactions, 1841, Plate VIII. fig. 37 o. 
