ON BRITISH FOSSIL REPTILES. 133 



The transverse processes disappear in the posterior caudal vertebras. The 

 chevron bones, of which three are preserved in the slab containing the six 

 caudal vertebree, exhibit the perforated character which distinguishes them 

 from those of the Cetiosaurus and of all existing Crocodiles and Lizards, not 

 excepting the Iguana, in which the haemapophyses are anchylosed at their 

 distal or spinal end only, and remain separate and articulated to two distinct 

 surfaces, at their proximal ends. The length of the superior and inferior 

 vertebral spines, and the shortness of the transverse processes, prove the form 

 of the tail to have been flattened laterally and of great breadth in the vertical 

 direction, at its basal portion at least. 



Ribs. — These appendages of the vertebral column are largely developed in 

 the thoracic abdominal region of the spine, and had the same two-fold con- 

 nexion with the vertebrae as in the other Dinosaurs and the Crocodilians. At 

 the anterior part of the costal region of the spine, the rib was joined by a 

 large head to a shallow cavity, situated at first on the side of the centrum and 

 then on the side of the neurapophysis ; and it was further articulated by a 

 tubercle to the extremity of the transverse process. In a certain number of 

 the anterior vertebrae, the neck of the rib was co-extensive with the transverse 

 process, and sometimes six or seven inches in length ; afterwards the neck of 

 the rib began to shorten, and the head to decrease in size, and to have its place 

 of articulation brought progressively nearer to the end of the transverse pro- 

 cess, until it finally disappeared, and the posterior ribs became appended to 

 the ends of the transverse processes. 



In the Iguana, as in other Lizards, the ribs have but one mode of articu- 

 lation, viz. to a simple tubercle developed from the side of the centrum. 



One of the largest double-jointed ribs of the Jguanodon, in the Mantellian 

 Collection (No. gjig)' is 46 inches in length. 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 with 

 the transverse process, 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 trun- 

 cated, 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-com- 

 pressed 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 dia- 

 meter ; 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 com- 

 pressed ; and its external margin, besides being bent backwards, is also deve- 

 loped 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 the upper part of their bony box is 

 almost wholly formed. 



Bones of the Extremities. 



Scapular Arch. — The scapula has not hitherto been discovered so asso- 

 ciated with other unequivocal portions of the skeleton of the Iguanodon as to 

 permit the characters of this bone in that species to be confidently recognised. 

 The bone (No. 194', Oniopliitc of Iguanodon, Mantell. Catal.) agrees with 



