282 BRITISH FOSSIL REPTILES. 



in PI. 7, figs. 1 and 2. The neck is less distinct from the tubercle and body than 

 in other ribs which seem to have been situated further back ; it expands more 

 gradually to the tubercular articulation ^vith the diapophysis, and is at this part 5 

 inches in breadth ; it bends with a deep oblique curve for about one fifth of its length, 

 and then is continued in a nearly straight line to its extremity : this is slightly expanded 

 and truncated, for the attachment doubtless of a bony sternal rib. The convex or 

 outer margin of the rib is bent backwards so as to overhang the sub-compressed shaft 

 of the bone along its upper or proximal third part. 



The proximal extremity of one of the ribs from the middle of the trunk of the 

 Horsham Iguanodon, presents an ovate head 2^ inches in the long diameter; the neck 

 is 7 inches long, straight, compressed, and topped by a well-marked tubercle, where it 

 joins the body of the rib. This part is also compressed ; and its external margin, 

 besides being bent backwards, is also developed in the contrary direction, so as to 

 assume the form of a slightly convex plate of bone 2 inches broad, attached at right 

 angles to the shaft of the rib, which it overhangs on both sides. This structure is 

 characteristic also of some of the ribs in the other Dinosaurs, and is interesting as 

 indicating the commencement of that peculiar development of the corresponding part 

 of the ribs in the Chelonian reptiles, by which, and their connexion with continuous 

 dermal ossifications, the lid of their bony box is almost wholly formed. 



In fig. 3, PI. 7, is given a view of the upper surface of the head, neck, and 

 tubercle, and expanded beginning of the shaft of a rib of an enormous Iguanodon, the 

 part so represented measuring 10 inches in a straight line. A ridge is developed from 

 the upper surface which, at the tubercle, expands into the overhanging plate of bone. 

 In a more posterior rib, figs. 4 and 5, the tubercle is more distinctly developed, and 

 continues so to be as the neck progressively shortens, as in figs. 6 — 10, PI. 7. Fig. 

 8 gives a view of an almost entire pleurapophysis, or " vertebral " rib, from about the 

 middle of the thoracic abdominal cavity, the length of which, in a straight line, from 

 the tubercle to the fractured end of the body, is 32 inches. The common form of the 

 body of the rib is that exemplified in the transverse section of the rib, given at fig. 1. 

 The number of thoracic abdominal vertebrae, supporting such free and more or less 

 elongated ribs, was probably about fifteen. 



Sacrum of the Iguanodon. Pis. 8, 9, 10, 11. (Half nat. size.) 



The facts ascertained relative to the structure of that part of the vertebral column, 

 answering to the " true vertebrae" in Human Anatomy, of the Iguanodon, had tended, 

 at the period of preparing my ' Report on British Fossil Reptiles,' in 1 840, to rectify the 

 ideas on the Laccrtian affinities of that reptile, suggested by its name, and had proved the 

 Iguanodon <to belong to a more highl}^ organized section of the then-defined Saurian 



