14 FOSSIL REPTILIA OF THE 



This increase is still more marked in the fourth sacral vertebra (fig. 4), 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 Hylgeosaurus 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 Hylseosaurus than in the Iguanodon, and probably indicate 

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



The base of the pleurapophysis or rib-element — taking the place and function of 

 an inferior transverse process in the Dinosaurian sacrum — may be discerned, wedged 

 into the interspace between the second and third sacral vertebras at pi 3, and again 

 between the third and fourth vertebras, at pi 4, fig- 2, T. V. 



A third portion of the sacrum of the Hylseosaurus, which escaped the cognizance 

 of the authors of the paper in the ' Philosophical Transactions ' for 1 849, is the 

 specimen No. 28,936, British Museum. This consists of the third sacral vertebra, 

 with part of the second and fourth anchylosed therewith, a great proportion of the 

 neural arch, and a small part of the left ilium being included in this very instructive 

 specimen. It is from the submerged Wealden of the Isle of Wight, and has been 

 subject, like many of the fossils from that locality, to a certain degree of attrition by 

 sea- waves on the beach. 



The pleurapophysis (fig. 3, pi 3), continued from the obliterated interspace between 

 the third and second vertebrae, quickly assumes the form of a broad and high plate, 

 compressed from before backwards, and again becoming thickened when it abuts 

 against the ilium (62). 



The diapophysis (fig. 4, a 3), arising from the side of the neural arch, seems to 

 form the upper part of the same broad, vertical, transverse wall of bone ; but the 

 suture between the pleurapophysial and diapophysial elements of this wall is clearly 

 traceable, extending from the base of the neurapophysis upwards and outwards. The 

 diapophysis at its upper part expands, and seems to bifurcate or abut against the side 

 of the base of the neural spine. This spine forms, at the part of the sacrum here 

 described, a continuous ridge of bone. 



The fractured outer border of the ilium has been rounded and water-worn to its 

 present form, which must not be taken as indicating its natural one. A large vacuity 

 was bounded by the ilium and the two contiguous diapophysial plates (fig. 3), as in 

 the sacrum of the Iguanodon : the large nerve-outlet, formed by the receding borders 

 of contiguous neural arches, and the middle part of the centrum, opens into the large 

 space above defined. 



