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XV. Additional Observations on the Osteology of the Iguanodon and Hylceosaurus. 
By Gideon Algernon Mantell, Esq., LL.D., F.R.S., F.L.S., 
Vice-President of the Geological Society, 8^c. 
Received January 15, — Read March 8, 1849. 
In the last memoir which I had the honour of placing before the Royal Society, 
allusion was made to the discovery of some remains of the Iguanodon that tended to 
elucidate the structure of certain parts of the skeleton of that gigantic terrestrial 
animal, which from the imperfect data previously obtained, had not been satisfac- 
torily determined. 
I now beg to submit to the consideration of the Society the results of a careful 
examination of these fossils, and of other illustrative specimens in the collections of 
my friends, in the hope that this addition to the previously recorded knowledge of 
the osteology of the Wealden Reptiles, wall be found to possess considerable physio- 
logical interest and importance. 
The fragmentary and isolated condition of the osseous relics found imbedded in 
the fluviatile deposits of the South-East of England, affords a suflScient excuse for 
error in the interpretation of a piece of bone, or in the reference of an entire bone to 
a particular genus or species. My own mistakes in this respect I shall unreservedly 
point out, and I would fain hope that other labourers in the same department of 
natural history, but of far higher authority, will not hesitate, after the noble example 
of the illustrious Cuvier, to make similar admissions ; for it is only by substituting 
truth for error, and facts for hypotheses, that correct principles of palaeontology can 
be established. 
The fossils in my possession consist of several bones of the extremities and pectoral 
arch, and of cei'vical, dorsal, and caudal vertebrae of the Iguanodon, equal if not 
surpassing in magnitude any previously discovered ; and portions of the sacrum of 
individuals of different ages ; together with dermal and other bones of the Hylaeo- 
saurus, Goniopholis, &c. The recent acquisition of some of these relics excited in 
my mind a desire to renew the attempt to construct the skeleton of the colossal reptile 
whose remains were first brought to light by my early geological researches in Tilgate 
Forest, I therefore repaired to the British Museum, and by the kind permission of 
Mr. Konig, re-examined many of the fossils described in my former works. I also 
availed myself of the liberality of Capt. Lambart Brickenden, Mr. Baber, Mr. Saull, 
&c., to inspect their several collections, and chisel out and figure or describe such 
specimens as threw light on the especial object of my present inquiries. 
In the difficult and tedious investigations necessary to arrive at any certain con- 
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