12 FOSSIL REPTILIA OF THE 



The Sacrum (T. V and VI). 



There is a portion of a sacrum of a small or young Dinosaur (T. VI, figs. 1 and 2, 

 No. ■££&, British Museum), which, in the form and proportions of the bodies of the 

 vertebrae, most resembles the present genus, and cannot be referred to either the 

 Megalosaurus or Iguanodon. It includes two entire and parts of two other vertebral 

 bodies, anchylosed together, and to the bases of the neurapophyses, which, as in the 

 Megalosaurus, are transferred to the upper and lateral parts of the interspaces of the 

 subjacent bodies. These are moderately, but regularly, contracted in the middle and 

 chiefly laterally, being more flattened below, where likewise each is traversed by a 

 longitudinal sulcus. At the middle of each lateral concavity there is a vascular perfo- 

 ration. I am uncertain which is the anterior part of this interesting series ; but, by 

 the analogy of the Megalosaurus, conclude that vertebra which supports the greatest 

 proportion of its neural arch to be posterior to the adjoining one which supports the 

 remaining small proportion. On this basis also I assume that the anterior sacral 

 vertebra is deficient, if we may allow five to the Hylaeosaur as to the other Dinosaurs. 



The second sacral vertebra, then, is here broken across the middle of the body, 

 exposing its solid minutely cellular central structure : its neural arch is too mutilated 

 for profitable description : its base rests nearly equally on the second and third sacral 

 bodies. The third neural arch, which exhibits a similar relative position, has its base 

 extended half way down the vertebral interspace; its strong transverse process, 

 diapophysis and pleurapophysis combined, extends outwards and forwards, and is at 

 first contracted, then expands both, transversely and vertically, most so in the latter 

 direction, and is twisted obliquely, so that the lower end is directed downwards and 

 forwards, and the upper and thicker end is bent obliquely backwards, until it meets 

 and becomes anchylosed to the anterior production of the transverse process of the 

 next vertebra behind : an elliptical space is thus produced, the axis of which is nearly 

 vertical, and into this space the neural canal opens ; the nerve being transmitted over 

 the middle of the body of the vertebra, as in the sacrum of the Megalosaurus and 

 Iguanodon. 



The upper and inner part of the base of the broad, oblique transverse process, or 

 sacral rib, abuts against the base of the spinous process. There is no appearance of 

 accessory spines or metapophyses, such as the sacrum of the Megalosaurus is com- 

 plicated with. 



The following are admeasurements of the present portion of the sacrum of the 

 Hylaeosaurus : 



