FOSSIL REPTILIA OF THE 



this surface was joined, if not anchylosed, the base of a bone, corresponding with that 

 which has been figured as the " horn " of the Iguanodon (z) 1 \ the surface on the radius, 

 like the co-adapted one on the base of the ' horn,' is 4 inches in long diameter, and 2 inches 

 4 lines in short diameter. 



The unsymmetrical character of this supposed ' horn' led me to infer that it was one 

 of a pair of bones, which I conjectured to be ' phalangeal.' 2 The rough flattened base of 

 the original specimen, on part of which the cellular osseous texture was exposed, I believed 

 to be due to the articular surface " having been chiselled or scraped away." 3 I now know 

 that it was a natural surface due to separation from a close syndesmotic and partially 

 anchylosed union with the distal end of the radius, as in the left antibrachial bones figured 

 in PL I, fig. 1. 



In the right radius of the Iguanodon, which has afforded the subject of the present 

 Monograph, this horn-like appendage is anchylosed, and stands out from the radial side 

 of the distal end like a process of the bone (PL II, fig. 1). 



The length of the detached radial spine in the left fore-limb is 6 inches ; the apex is 

 not quite entire ; the thenal surface (PL I, fig. 1, z) is less convex across and more 

 convex lengthwise than the anconal surface (PL II, fig. 1, z). This surface is strongly 

 convex transversely, slightly concave lengthwise, and is smooth along its distal half; it 

 is roughened by thick and strong longitudinal ridges at its proximal third, and these are 

 less developed at the corresponding part of the thenal surface. 



The vascular channels indicate, as in a claw-phalanx, the system of supply of horny 

 matter sheathing the bone. 



The formidable spine, supported by this bony core, projected inward or from the radial 

 side of the radius, with its distal border at right angles with the long axis of the bone, 

 the proximal border (z) passing more obliquely to the apex of the spine-core. 



The right ulna shows an exostosis at the back part of the shaft, near the base of the 

 olecranon. Such instances of disease in Mesozoic reptiles are rare. 



There is a slight difference in the shape of the proximal end of the right radius, 

 which, nevertheless, belonged to the same individual Iguanodon, as the left one above 

 described : the humeral surface, or 'head,' is 3 inches 5 lines by 2 inches 9 lines; the 

 principal ulnar surface is 2 inches 3 lines by 1 inch 6 lines. The narrower surface for 

 the ulna, extending upon the ridge-like process, with the digital depression dividing it 

 from the broader ulnar surface, show the same characters, as at r, s, figure 3, PL I. 



Fracture of the shaft of the right radius (fig. 5) shows a compact bony wall, 6 to 7 

 lines in thickness, surrounding a finely cancellous central tract : the shaft is sub-trihedral, 

 approaching the cylindrical form prior to the distal expansion. 



' Mantell, ' Illustrations of the Geology of Sussex,' 4to, 1827, p. 78, pi. xx, fig. 8. 



2 'Monograph on the Fossil Reptilia of the Wealden Formation,' 4to, 1855, p. 46. 



3 lb., ib. 



