XVIL On a Dorsal dermal Spine of the Hylceosaurus, recently discovered in the 
Strata of Tilgate Forest. 
By Gideon Algernon Mantell, Esq., LL.D., F.R.S., F.L.S., 
Vice-President of the Geological Society, 8fc. 
Received June 13, — Read June 13, 1850. 
In the highly interesting and unique specimen of part of the skeleton of the Hylseo- 
saurus discovered by me in 1832, and now preserved in the Gallery of Organic Re- 
mains of the British Museum, the most striking peculiarity is a series of large thin 
angular processes extending nearly parallel with the left side of the vertebral column. 
These bones vary in length from 4 to 17 inches, and in form from a somewhat obtuse 
to an isosceles triangle : there are six or seven in a connected line, and several others 
detached and dispersed in the block. They terminate distally in a blunt apex, and 
are expanded at the base in the antero-posterior diameter, but unfortunately in every 
instance the proximal end is so imperfect that their mode of attachment to the other 
parts of the skeleton is not clearly demonstrable. 
From the general resemblance of these processes to the dermal bones imbedded in 
the surrounding stone, I was induced to consider them as the remains of a dorsal 
crest formed by a series of erect dermal plates or spines, which extended along the 
back of the Hylseosaurus, in the same manner as the cartilaginous scaly dorsal 
fringe in the Iguana ; an opinion which appeared to be corroborated by some de- 
tached specimens of a similar character that were subsequently discovered^. 
Professor Owen, however, whilst admitting the probability of that suggestion, con- 
sidered it more likely that the bones in question were abdominal ribs, remarking that 
“the want of symmetry, and the difference in size and form, in the four succeeding 
spine-shaped plates, agreed better with the costal than with the dermal hypothesis-f',” 
Mr, Broderip and other naturalists have regarded this opinion of Professor Owen as 
conclusive:}:. 
For reasons fully stated in my former memoirs on the Wealden Reptiles, it ap- 
peared to me highly improbable that these spines could have belonged to the costal 
system. In 1848 I had an opportunity, for the first time, of making sections of a 
specimen for microscopical examination, and I then found the internal structure to 
* A comparison of these spines with the dermal crest of the Cyclura, is given in my original memoir on the 
Hylaeosaurus ; Geology of the South-East of England, 1832. 
t British Association Report on Fossil Reptiles, by Professor Owen, 1841, p. 116, 
J Zoological Researches, Chapter on Reptiles, 
