358 BRITISH FOSSIL REPTILES. 



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 

 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 vertebrse in the portion of 

 the skeleton of the Hylseosaurus (PI, 35) : 



Length, or antero-posterior diameter of centrum . 

 Height, or vertical diameter of its articular end . 

 Breadth, or 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.f 

 The vertebrae of the Hylaeosaurus differ from those of the Iguanodon in their greater 

 relative transverse diameter, and in the greater 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 Strep tospondy] us in the sub-biconcave character of both the articular 

 ends of the centrum : 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. 



The Sacrum (Pis. 36 and 37). 



There is a portion of a sacrum of a small or young Dinosaur (PI. 37, fig. 1, 

 No. 2W1, 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 



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

 •f Compare with those given at p. 335. 



