ox BRITISH FOSSIL REPTILES. 113 



cess is developed from the side of the neurapophysis, and is supported by a 

 pyramidal underprop, extending upwards and outwards from the anchylosed 

 base of the neurapophysis. There is a large, deep and smooth depression on 

 each side of the base of the transverse process. Tlie anterior surface of the 

 neural arch, above the anterior oblique processes, is traversed by a vertical 

 ridge, on each side of which there is a shallow depression*. The spinous 

 process is of unusual thickness, its transverse breadth at the base measures 

 1 inch : this modification may probably relate to the support of great dermal 

 spines. The spinal canal in the dorsal vertebrae is cylindrical, and expanded 

 at both extremities ; its diameter at the middle is 7 lines, at the expanded out- 

 lets 10 lines, in a posterior dorsal or lumbar vertebra. Here the bases of the 

 neurapophyses begin to shorten, and leave a small proportion of the upper 

 surface of the centrum uncovered at both ends, chiefly at the posterior end. 

 The following are dimensions taken from three of the vertebrae of the Hy' 



IcEQsaurus: — Second Fourth n/ij.i 



Middle 

 conspicuous conspicuous . , 



_ • 1 • 1 dorsal, 



cervical. cervical. 



In. Lin. In. Lin. In. Lin. 



Antero-posterior diameter of body . 1 10 2 2 2 9 



Vertical diameter of its articular end 16 2 6 



Transverse diameter of its articular end 2 2 2 3 



Transverse diameter of middle of body 2 



The differences between the vertebras of the Hijlceosaurus and Megalo- 

 saurus have been already pointed out, and are further shown in the admea- 

 surements given above : the vertebrae of the HylcBosaurus differ from those 

 of the Iguanodon in their greater transverse diameter, and in the breadth of 

 their under part ; those of the Iguanodon are flatter vertically along their 

 whole sides, which converge to a narrower ridge at the under part. The 

 vertebrae of the Hylceosaurus differ from those of the Streptospondylus in 

 the sub-biconcave character of the articular ends of the centrum, and in its 

 comparative shortness and thickness : the separated neural arch might be 

 distinguished from that of the Streptospondylus by the simplicity of the sup- 

 porting buttress of the transverse process; and, although equal in height, yet it 

 is superior in the expansion and strength of the platform and spinous process. 



Sacrum, — There is a portion of a sacrum of a small or young Dinosaur 

 (No. 2484> Mantellian Collection), which, in the form and proportions of the 

 bodies of the vertebrae, most resembles the present genus, and cannot be re- 

 ferred to Megalosaurus or Iguanodon. It includes two entire and parts of 

 two other vertebral bodies, anchylosed together, and to the bases of the neu- 

 rapophyses ; which, as in the Megalosaurus, are transferred to the upper and 

 lateral 2)arts of the interspaces of the subjacent bodies. These are moderately, 

 but regularly, contracted in the middle and chiefly laterally, being more flat- 

 tened below, where likewise each is traversed by a longitudinal sulcus. At 

 the middle of each lateral concavity there is a vascular perforation. 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 great- 

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

 supports the remaining small proportion. On this basis also 1 assume that the 

 anterior sacral vertebra is deficient, if we may allow five to the Hylaeosaur us 

 to the other Dinosaurs. 



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



* This description is taken from Nos. 2586 and 2125, parts of tlie same vertebra iu the 

 Mantellian CoUectiou. 



1841. 1 



