360 BRITISH FOSSIL REPTILES. 



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 Hyljeosaurian series of the British 

 Museum, where it still bears the ticket and numbers, oWt' under which its nature was 

 first made known. f 



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

 (PI. 36) are figured as "belonging either to the Hylseosaurus or Iguanodon" (p. 301). 

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

 comparison with which the sacrum (PI. 36) might have been determined, left the authors 

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

 museum suffices to difi"erentiate the present specimen. It consists of the confluent 

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

 Pis. 8—11 of the description of the Iguanodon (pp. 282—288). 



The body of the second sacral vertebrce of the Hylaeosaurus (PI. 36, fig. 2, 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. 



This increase is still more marked in the fourth sacral vertebra (fig. -j), which is 

 also longitudinally, but more widely, channelled along its under surface. 



The breadth, as compared with the length, increases in the fifth sacral vertebra 

 (5), shown to be the last, as in the Iguanodon, by the terminal articular surface 

 for the first caudal vertebra. Like the preceding centrums, that of the fifth sacral 

 vertebra in the Hylseosaurus is relatively broader and flatter below than in the 

 Iguanodon : but the lateral compression beneath the wide outlets for the nerves, 

 usually intervertebral in position in other reptiles, is well marked. These outlets are 

 relatively wider in the Hylaeosaurus than in the Iguanodon, and probably indicate 

 greater activity, and a swifter rate of motion, in the smaller herbivorous Dinosaur. 



The 1)ase of the pleurapophysis or rib-element — taking the place and function of 



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



