THE IGUANODON AND HYLiEOSAURUS. 
275 
Sacral and caudal vertebras, Plate XXX. — The most important and novel announce- 
ment in relation to the osteology of the Wealden reptiles in Professor Owen’s Reports, 
was the exposition of the structure of the sacrum in the three remarkable extinct genera 
of his order Dinosauria; namely, the Megalosaurus, Hylaeosaurus, and Iguanodon ; 
a peculiarity of mechanism which had escaped the penetration of all previous ob- 
servers. No one appears to have suspected that in these reptiles the pelvic arch was 
composed of a greater number of anchylosed vertebrse than in the living Saurians ; 
and that the position of the neural arches was transposed from its usual place over 
the middle of the body of the vertebra, to the ossified intervertebral spaces formed 
by the anchylosis of the contiguous vertebrae ; the foramina for the transmission of 
the sacral nerves from the spinal chord being situated above and behind the middle 
of the body (see Plate XXX. figs. 15, 16)*. 
Fragments of the pelvic arch, consisting of the body of one sacral vertebra, with a 
portion of the contiguous bones anchylosed to each extremity, are not uncommon in 
the Wealden deposits; and so long since as 1826, Sir R. Murchison transmitted to 
Baron Cuvier a specimen of this kind (from Loxwood in Sussex-f^), with several 
lumbar and caudal vertebree. Upon these relics the illustrious founder of palaeonto- 
logy only remarked, that the united bodies of the vertebrae “ seem to indicate that 
the animal to which they belonged made such feeble use of its tail that the caudal 
vertebrae were occasionally anchylosed together.” Neither did the magnificent spe- 
cimen of the sacrum of the Megalosaurus, consisting of a series of five united ver- 
tebrae, made known by the present Dean of Westminster in 1824, suggest the correct 
interpretation of this part of the skeleton of the Dinosaurians. The announcement 
of Professor Owen was therefore to me of especial interest, since it elucidated the na- 
ture of many fossils in my collection which had previously been undeterminable. 
The present investigation rendering it necessary to acquire an accurate idea of the 
characters of the vertebrae composing the pelvic arch of the Iguanodon, I obtained 
permission of Mr. Saull to have the fine specimen of a sacrum in his museum (de- 
scribed in Report of Brit. Assoc, p. 131), more completely developed at my own ex- 
pense, as its true characters were in some measure obscured by the coating of hard 
calcareous grit with which, as is generally the case with the Isle of Wight Wealden 
fossils, it was partially invested. This interesting and instructive relic is figured as 
it now appears in Plate XXVI. ; half the natural size in linear dimension. 
This sacrum consists of six anchylosed vertebrse (not oijive as described in the 
Reports on Brit. Foss. Reptiles, p. 130), with the right iliac bone attached. The re- 
lative size and proportions of the several bones composing the sacral arch are now 
well displayed. The body of the first or anterior vertebra (Plate XXVI. 1) is large, 
strong, and expanded, forming a powerful buttress in front ; the bodies of the two 
posterior vertebrse (Plate XXVI. 5, 6) are likewise large and strong ; but the second, 
third, and fourth, are constricted laterally in the middle (Plate XXVI. 2, 3, 4), and 
* See Reports on Brit. Foss. Reptiles, 1842, p. 105. 
t Geological Transactions, vol. ii. (New Series), p, 105, Plate XY. figs. 4, 6. 
