WEALDEN FORMATIONS. 25 



Whether the bones in question be dorsal spines or abdominal ribs, they have 

 evidently been displaced from their natural position in the partial disarticulation of the 

 entire skeleton (T. IV) prior to its immersion in the mud that has been subsequently 

 hardened around it; but the degree of displacement has not been greater in the one 

 case than in the other. 



In offering, with due diffidence, a choice of opinions respecting the nature of these 

 singular bones, I have been actuated solely with the view of accelerating the acquisition 

 of the true one ; which, it is obvious, will be more likely to be attained by the choice 

 being present to the mind of subsequent fortunate discoverers of these remains of the 

 Hylseosaurus, than if they were solely preoccupied by the hypothesis of the dorsal 

 fringe. For example, it may lead to more careful noting of the constancy or otherwise 

 of the unsymmetrical inclination of the convex margin of the spine, and whether they 

 form, or are disposed in, pairs; which, on the costal hypothesis, may be expected, in 

 the event of another skeleton being discovered. 



The peculiarly unsymmetrical figure of these problematical bones is strikingly 

 shown in the specimen (T. IX, figs. 2 and 3, No. 28,861) now in the British Museum, 

 discovered in the same quarry in Tilgate Forest, whence the above-described part of 

 the skeleton of the Hylaeosaurus was obtained. 



It is a long triangular plate of bone (fig. 3), thickened at the base, becoming rapidly 

 compressed or flattened beyond it, and gradually decreasing in thickness and breadth 

 to the apex. Both the apex and one angle of the base have been broken away ; but 

 the bone can hardly have been under 8 inches across the base, and 15 inches in total 

 length. 



The base is surrounded by a low, obtuse, thick ridge ( a ), and is excavated by an 

 irregular angular depression (b), the sides of which extend below or beyond the 

 boundary ridge, at c and d ; these productions not being opposite, but adding to the 

 general oblique and unsymmetrical character of the apparently articular surface. 



The body of the bone is moderately convex on one side (T. IX, fig. 3), and corre- 

 spondingly concave on the opposite side, at the basal two thirds of its extent, beyond 

 which the surface becomes convex transversely, but retaining its longitudinal 

 concavity (fig. 2). 



Several coarse vascular canals open upon and groove for a greater or less extent 

 the outer surface of the bone, indicative of the periosteum being connected with a 

 corium producing a thick epidermal covering ; and this feature much inclines me to 

 regard the bone as a true dermal spine. On the same hypothesis, the groove between 

 the boundary ridge of the base and the projecting parts of the border of the basal 

 depression, may have served for the implantation of dermal muscles, regulating the 

 position of such spine. 



But if these osteodermal spines formed a single series along the mid-ridge of the 

 back, as the purely epidermal spines do in the Cyclura, they must have overlapped each 



4 



