WEALDEN FORMATIONS. 11 



novelties, and most important additions, to be submitted to the members of the Palseon- 

 tographical Society, and other cultivators of geology, in my forthcoming ' Monographs 

 on the Fossil Reptilia of Great Britain,' of which the ' Reports ' to the British Asso- 

 ciation, in 1840 and 1841, were the basis. 



In some particulars I have been aided, and in a few illustrations anticipated, by 

 the labours of zealous contemporaries. Two associated authors, taking advantage of 

 the indications given in my ' Report,' obtained Mr. Saull's permission to examine the 

 sacrum of the Iguanodon which I had discovered, and had a drawing taken of it, which 

 they published in the ' Transactions of the Royal Society,'* confirming, in the main, 

 my description, but describing an attached lumbar vertebra, as a sixth sacral one. 



As the characters of the order Dinosauria were mainly founded on this specimen 

 four plates have, in this ' Monograph,' been devoted to the illustration of its remarkable 

 structure. 



Tab. Ill gives a view of the under surface, of half the natural size in linear 

 admeasurement. 



The last of the lumbar series, l, upon the interspace between which and the first 

 true sacral vertebra the neural arch of that vertebra, n 1, Tab. IV, has advanced, has 

 thereby become confluent with the sacrum proper, characterised by the junction of its 

 transverse processes with each other, and with the iliac bones. The confluent lumbar 

 vertebra has a much broader centrum or body, , than that of the contiguous sacral 

 vertebra, especially at its middle part, which presents a subquadrate transverse section, 

 the sides being vertical, excavated near the neurapophysis, and meeting the under 

 surface at a right angle : the under surface is convex transversely, especially at its 

 middle part, concave longitudinally. The anterior articular surface is quadrate, with 

 the angles rounded off, and is broader than it is deep : it is slightly convex vertically, 

 flat transversely, except near the periphery, which is convex : some remains of its 

 water-worn and mutilated neural arch, showing the normal relation of its piers to the 

 centrum, and its partial anchylosis to the advanced neural arch of the first sacral 

 vertebra, are shown at % n, fig. 1, Tab. VI : the antero-posterior extent of the piers is 

 short ; the neural interspace between them and those of the first sacral vertebra is 

 wide. 



The body of the first proper sacral vertebra, s 1, Tab. Ill, differs from the fore- 

 going by its sudden decrease in transverse diameter, especially at its middle part, the 

 sides being concave lengthwise, and with the under surface compressed and produced 

 into a low ridge. In consequence of the advanced position of its neural arch, the first 

 pair of sacral nerves pass over the upper surface of the centrum about one third from 

 its hinder end, and deeply groove that surface in their passage ; the fore part of the 

 advanced arch of the succeeding vertebra rests upon and has coalesced with the 

 expanded hinder end of the first sacral vertebra. The transverse processes of this 



* ' Philosophical Transactions,' 1849, p. 275, pi. xxvi. 



