ON BRITISH FOSSIL REPTILES. 129 



wide and deep depression, and the neurapophysis extends backwards to form 

 the posterior articular processes which project 1^ inch beyond the hind sur- 

 face of the centrum. The antero-posterior extent of the neurapophysial plat- 

 form is 6 inches ; the dimensions of the oval articular surfaces of the oblique 

 processes are 2 inches by 2^ inches ; the inferior margins of the posterior 

 processes are separated by a groove. A smaller anterior ridge extends along 

 the anterior part of the neurapophysis. The base of the spinous process ex- 

 tends from the posterior triangular interspace of the oblique processes forwards 

 and downwards along the curve of the supporting platform ; the thickness of 

 the spine, which is 1 inch at the posterior part of the base, gradually dimi- 

 nishes towards the fore part of the vertebra. The anterior oblique processes 

 form the sides of an angular depression in front of the base of the spine. 



The spinal platform of the Iguanodon differs from that of the Megalosau- 

 rus in its greater relative antero-posterior extent, arising from its being ex- 

 tended further back ; the platform is also raised higher above the centrum. 



No. ^. Mantellian Collection, is a dorsal vertebra, posterior in situation 

 to the preceding, and from an individual of the same size. The neural arch 

 is anchylosed, but the sutural line is obvious. The surface for the head of the 

 rib on the side of the neurapophysis is smaller, and a transverse process begins 

 to be developed above that surface, throwing its aspect somewhat downwards. 

 The costal surface is separated in this as in the preceding vertebra by a strong 

 vertical ridge or buttress from the wide depression below the posterior part 

 of the base of the spine. The angle between the oblique processes is rather 

 more open. The spinous process of this vertebra, almost entire, is detached 

 from the neural platform, but is cemented to the same mass of stone: it is 

 9 inches in height and 3 in breadth, or antero-posterior extent; the summit 

 is, however, wanting. The following are other dimensions of the present ver- 

 tebra :— ■ In. Lines. 



Antero-posterior extent of the body 3 10 



Vertical diameter of the body 3 9 - 



Transverse diameter of the body 3 7 



The sides of the centrum are as usual concave lengthwise, but are slightly 

 convex vertically, and converge to the lower surface, which is formed by an 

 obtuse ridge. 



In a dorsal vertebra of the Horsham Iguanodon in Mr. Holmes's collection, 

 from apparently the middle of the back, the spinous process, which is 8 inches 

 in length, expands gradually in breadth and thickness as it ascends to its trun- 

 cated summit, the antero-posterior diameter of which is 4 inches, its trans- 

 verse diameter or thickness being 1 inch 6 lines. 



In a series of eight posterior dorsal vertebrae, measuring together 1 foot, 

 and consequently from a young Iguanodon in Mr. Holmes's collection, the 

 spinous process of the most anterior one is, in antero-posterior diameter, 7 

 lines, but increases in the other vertebrae to 15 lines, which shows a somewhat 

 rapid change of character. 



Sacral Vertebrce. — The highly remarkable and characteristic structure of 

 the sacrum of the Megalosaurus, and the strong indications of close affinity 

 between this gigantic carnivorous reptile and the still more colossal herbivo- 

 rous Igxianodon, wliich the structure of their costal vertebrae, of their ribs, 

 and of the larger bones of their extremities afford, made it very desirable to 

 ascertain whether the Iguanodon deviated in the same manner from other 

 Saurians, existing and extinct, in the extent and structure of the sacral region 

 of the spine. 



The collection of the remains of the Iguanodon in the British Museum does 

 1841. K 



