506 BRITISH FOSSIL REPTILES. 



The third or radial side of the shaft of the ulna has been somewhat crushed in, but 

 seems to have been rather convex transversely, and is less sharply defined than are the 

 other tvro surfaces. The thick rounded border between it and the hinder surface 

 gradually subsides at the lower fourth of the shaft, and both blend into the somewhat 

 flattened rough surface opposite the articular one at the distal expansion of the bone. 



The thick rounded border between the ulnar and radial sides of the shaft contracts about 

 the lower fifth of the bone, inclines forward, and extends into the beginning of the rugous 

 margin {k, k', fig. 1), which defines, by a convex curve, the lower or distal end of the bone. 



The non-articular surface of this expansion is smooth anteriorly, where the radial facet 

 of the shaft terminates ; but is roughened by oblong tuberosities posteriorly, where the 

 hinder facet of the shaft is lost upon it. 



The articular surface for the distal expansion of the radius is of a crescentic shape, 

 with the anterior horn the longest. It is rough and irregular on the surface, indicative 

 of the ligamentous nature of the union. The smooth ulnar surface of the shaft terminates 

 ill the hollow of the crescent. The anterior horn extends 4 inches 3 lines above the distal 

 end of the bone ; the posterior horn 2 niches 6 lines above the same end. The general 

 breadth of the syndesmotic surface is about 2 inches, contracting at each end of the 

 crescent. 



The compact bony wall of the ulnar shaft is from 6 to 9 lines in thickness ; the fine 

 cancellous centre, of an oval form in transverse section (fig. 4), is 1 inch 3 lines by 10 lines 

 in its diameters. 



In general shape, in the better definition of the joints for the humerus and radius, 

 and in the development of the olecranon, the ulna of the Iguanodon resembles that of the 

 larger living Lacertia more than that of the Crocodilia. From the idna of the Iguana 

 and of the large Nilotic Monitor it differs in the greater relative strength and more trihe- 

 dral figure, the shaft of the ulna being compressed and two-sided in the smaller recent 

 Lizards. There is the same concavity at the proximal part of the ulnar surface of the 

 bone ; but it seems relatively deeper in the Monitor. The chief diSerence in the 

 Iguanodon is the thick tuberous extension on the radial side of the radial articulation, 

 from which is continued that border which divides and defines the posterior and radial 

 surfaces of tlie shaft. 



Radius. Plate 46, 54. 



The length of this bone is IG inches;^ the greatest diameter of the proximal end 

 (fig. 3) is 4 inches ; of the distal end, from the upper border of the spur-surface (fig. 1, vi) 



I This appears to be its length in the Maidstone Iguanodon, but one end is covered by a crushed 

 vertebra. 



