268 PETRIFACTIONS AND THEIR TEACHINGS. CHAP. III. 



many fragments of ribs, and a few with the spinal end 

 entire : the magnificent specimen on the shelf to the right of 

 the group of caudal vertebrae, is the largest example of this 

 part of the Iguanodon that has come under my observation. 

 It was broken into numerous pieces in extracting the block of 

 sandstone in which it was imbedded, and these were chiselled 

 out singly, and connected together, and now form the finest 

 bone of this kind collected in Tilgate Forest ; though but a 

 portion of the entire rib, it is forty-six inches in length, and 

 five inches wide at its greatest breadth. 1 The ribs of the 

 Iguanodon have very rarely been found in connexion with 

 the vertebrae ; but the two-fold articulation of the costal pro- 

 cesses by means of a tubercle and the head, as previously 

 manifested in the structure of the anterior dorsal vertebrae, 

 are characters which enable us to recognize the detached ribs 

 of the gigantic herbivorous saurian. In the anterior ribs the 

 head is large, and of an ovate form ; the neck is very long, as 

 may be seen in several of the specimens in the collection, de- 

 creases progressively in the middle region of the spine, and 

 finally disappears : the posterior ribs being attached to the 

 ends of the transverse processes. This construction of the 

 costal elements corresponds with that of the crocodiles ; in 

 the lizards the attachment of the ribs to the vertebrae is by a 

 single tubercle 011 the side of the body of the vertebra. 



Sacral Vertebrce. The most important and novel feature 

 in relation to the osteology of the Wealden reptiles enun- 

 ciated in Professor Owen's Reports, was the remarkable struc- 

 ture of the Sacrum in the three extinct genera of Dinosau- 

 rians ; namely, the Megalosaurus, Hylaeosaurus, and Igua- 

 nodon ; a character first observed in a fine specimen consisting 

 of six vertebrae, with portions of the two iliac bones attached, 

 in the interesting collection of W. D. Saull, Esq. of Aldersgate 

 Street. 2 No one had previously suspected that in these rep- 

 tiles the pelvic arch was composed of more than two anchy- 

 losed vertebrae, as in the living Saurians (see ante p. 167), 

 and that the neural arches were transposed from their usual 

 place over the middle of the bodies of the vertebrae, to the 

 ossified intervertebral spaces formed by the anchylosis of the 



1 Several portions of ribs are figured in my " Fossils of Tilgate 

 Forest," PI. XI. 



2 See " Reports on Brit. Foss. Reptiles," 1842, p. 105. 



