THE IGUANODON AND H YL^EOSAURUS. 
293 
Saurian. Moreover, the nerves escaping through the intervertebral foramina of this 
region are larger than those of the dorsal segment of the spine, as they contribute to 
the formation of the lumbar and sacral plexuses ; the vertebral foramen would pro- 
bably also be wider, since the spinal chord enlarges in that region to form the pos- 
terior expansion or ganglion of the sinus rhomboidalis, which extends through the 
anterior half of the canal of the sacrum : the expanded bases of the neural laminae 
would therefore leave a portion of the centrum uncovered mesially, to form the floor 
of the canal and support directly the medulla spinalis. 
I can perceive no difference between the posterior dorsal or lumbar vertebrae 
(No. 2133 , 2115 )* assigned by Professor Owen to the Cetiosaurus brevis, atidthat last 
described as corresponding in some respects to the fifth dorsal in the spinal column 
of the Crocodile, than a diminution in the relief of the buttress supporting the trans- 
verse process. In No. 2115 the neural arch is broken away, and the tract of the 
centrum left uncovered behind to form the floor of the intervertebral foramen, is of 
greater extent than in No. 2133 , indicating a more posterior situation in the vertebral 
series. The approach to the quadrangular form of the body of this vertebra is no 
proof whatever of a specific and still less of a generic distinction ; otherwise the first 
sacral vertebra, which is more decidedly quadrate, if found separate, would be 
equally entitled to a generic value ; but its association, in the sacrum from Mr. Saull’s 
collection (Plate XXVI.), with other vertebral bodies of a very dissimilar character, 
and with the ilium of the Iguanodon, prevents our falling into an error of such 
magnitude. We may therefore reasonably conclude, that these vertebrae, to wit, 
Nos. 2133 , 2 155 , belong to the Iguanodon, and that the latter, in the form of the body, 
approached the first sacral, and was one of the proper lumbar series. The vertebra. 
No. 2109 , attributed in the above-mentioned report to the (so-called) second species 
of Cetiosaurus found in the Wealden formation {C. brachyurus), is also a posterior 
dorsal or lumbar vertebra of the Iguanodon ; the neural arch is much mutilated. The 
only other element of the skeleton of that species is a caudal vertebra. No. 2161 , 
which also belongs to the Iguanodon ; being in fact one of the most anterior of the 
caudal series, and contrary to the character of the genus to which it was referred, it 
presents one of the most interesting and instructive examples of the rough surface on 
the sides of the upper aspect of the centrum, left by the removal of the unanchylosed 
neural arch. The so-called Cetiosaurus brevis being thus founded only on two ver- 
tebrae which belong to the Iguanodon, must be expunged from the list of extinct 
reptiles. 
The angular posterior caudal vertebrae referred in the Report on British Reptiles, 
to the Cetiosaurus brevis, I am also inclined to assign to the Iguanodon for the fol- 
lowing reasons; — 1st, a similar vertebra, as far as can be ascertained, exists in the 
Maidstone specimen, and in this case an admixture of bones of distinct animals 
cannot even be suspected; 2ndly, the numerical ratio of the vertebrae of this kind 
* Mantellian Collection in the British Museum. 
MDCCCXLIX. 2 Q 
