WEALDEN FORMATIONS. 



13 



Length of the body of the third vertebra 



Breadth of its articular end 



Breadth of its middle part 



Breadth of its inferior groove . 



Length of the transverse process 



Antero-posterior diameter of the middle of process 



Vertical diameter of base of process 



Vertical diameter of expanded extremity 



From the lower part of centrum to the origin of the spinous proces 



In. 



Lin. 



2 







2 







1 



4 







4 



1 



10 







4 



1 



6 



3 







2 



6 



The spines appear to be anchylosed into a continuous ridge. The anterior surface 

 of the transverse process appears undulated by wide shallow depressions and interven- 

 ing elevations. 



The authors of a paper in the 'Philosophical Transactions' for 1849, who pre- 

 ceded me in the publication of the figures of the sacrum of the Iguanodon, first 

 discovered by me in the collection of the late Mr. Saull, and described in my ' Report 

 on British Fossil Reptiles/* state that "the sacral fragment referred to the 

 Hylaeosaurus by Professor Owen cannot at present be found." 



The fragment in question is the one above described. It has never, according to 

 my observation, been absent from its place in the Hylaeosaurian series of the British 

 Museum, where it still bears the ticket and numbers, -ggfe, under which its nature was 

 first made known. f 



In the paper in the ' Phil. Trans./ above cited, the four confluent sacral vertebras 

 (T. V) are figured as "belonging either to the Hylaeosaurus or Iguanodon" (p. 301) 

 The apparent inability to recognise the specimen of Hylaeosaurian sacrum, No. 2484, by 

 comparison with which the sacrum (T. V) might have been determined, left the authors 

 in the above state of doubt ; yet the unequivocal Iguanodon's sacrum in Mr. SaulFs 

 museum suffices to differentiate the present specimen. It consists of the confluent 

 bodies of four sacral vertebrae, answering to those marked s 2, s 3, * 4, and « 5, in 

 T. Ill of my ' Monograph on the Iguanodon.' 



The body of the second sacral vertebrae of the Hylaeosaurus (T. V, 2) is carinate 

 below, as in the Iguanodon. Above it is smoothly excavated to form the floor of a 

 capacious neural canal (fig. 1, n ), whence the nerves escaped, passing over the centrum, 

 in consequence of the blocking up of the vertebral interspace by the articulation there 

 of the shifted neural arch. 



The third sacral vertebra (3 ) is not carinate below, as in the Iguanodon, but 

 grooved along the middle line, and the increase of breadth is relatively greater in the 

 centrum. 



* * Reports of the British Association,' volume of 1842, pp. 129—131. f lb., pp. 113, 114. 



