WEALDEN FORMATIONS. . 11 



the axis of the vertebra, expanding transversely or outwardly, until it has attained a 

 height equal to that of the centrum ; there it expands into a broad and flat plat- 

 form (d), from the middle line of which the broad spine (««) is developed. A vertically 

 compressed but strong diapophysis ( d ) is developed from the side of the neurapo- 

 physis, and is supported by a pyramidal underprop ( t ), extending upwards and 

 outwards from the anchylosed base of the neurapophysis. There is a large deep and 

 smooth depression {p ) on each side of the base of the diapophysis. The anterior 

 surface of the neural arch, above the anterior oblique processes, or prezygapo- 

 physes (z), 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 vertebra? is cylindrical, and 

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

 outlets 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 in the portion of 

 the skeleton of the Hylaeosaurus (T. IV) : 



^ert 



ebra 



Vertebra 



Middle 



No. 



4. 



No. 6. 



dorsal. 



In. 



Lin. 



In. Lin. 



In. Lin 



1 



10 



2 2 



2 9 











1 6 



2 6 



2 







2 2 



3 















2 



Antero-posterior diameter of centrum 

 Vertical diameter of its articular end 

 Transverse diameter of its articular end 

 Transverse diameter of middle of centrum 



The differences between the vertebrae of the Hylaeosaurus and Megalosaurus have 

 been already pointed out, and are further shown in the admeasurements given above. 

 The vertebrae of the Hylaeosaurus 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 Hylaeosaurus differ from those of the Streptospondylus 

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

 parative shortness and thickness. The separated neural arch might be distinguished 

 from that of the Streptospondylus by the simplicity of the supporting buttress of the 

 transverse process ; and, although equal in height, yet is superior in the expansion 

 and strength of the platform and spinous process. From the vertebrae of the Poikilo- 

 pleuron, an oolitic Saurian of about the same bulk, those of the Hylaeosaurus differ 

 in their more compact osseous structure, and in the absence of the large cells that 

 characterise that structure in the vertebral bodies of the Poikilopleuron. 



* This description is taken from Nos. 2586 and 2125 parts of the same vertebra in the British Museum. 



