WEALDEN FORMATIONS. 7 



antero-posterior extent. The zygapophyses, z , /, dimmish in size, and their articular 

 surfaces become more vertical as the vertebrae approach the pelvis. The left pleur- 

 apophyses, pi, pi, of the first five of the imbedded vertebras, or those succeeding the 

 three cervicals, seem to show a progressive and rapid augmentation of length, indicative 

 of their formation of the fore part of the thorax, but the extremities of all their ribs 

 appear to have been broken off : the inner surface, which is exposed, shows a longitu- 

 dinal groove ; traces of ribs continue to the seventeenth vertebra. There are 

 impressions of spines of three vertebrae, beyond* this, before we come upon the blended 

 mass of the pelvis and hind foot. Before describing the pelvis, some notice is required 

 of the peculiarities of those elements of the cervical and dorsal vertebrae which, from 

 their development and retention of their primitive separation, are usually regarded 

 as distinct bones, called " ribs." 



Ribs of the Iguanodon. Tab. II. 



These appendages, or elements, of the vertebral column are present throughout a 

 great proportion of its extent, but become anchylosed, and reduced to the character 

 and function of transverse processes in the lumbar, sacral, and caudal regions. They 

 are free and largely developed in the thoracic-abdominal region of the spine, at the 

 fore-part of which they have the same two-fold connexion with the vertebras as in the 

 Crocodilians. In the cervical region the rib is articulated by a " head,'' supported on 

 a long neck, to a short sessile inferior transverse process or " parapophysis," and by a 

 large "tubercle" to a superior transverse process or " diapophysis ;" a portion of a 

 cervical rib is slightly disarticulated and turned forwards in one of the hinder cervical 

 vertebras of the young Iguanodon, figured in Tab. I, fig. 2, so as to show most clearly 

 the nature of this two-fold articulation ; as the ribs increase in length, at the fore-part of 

 the thorax, each is 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, as at Tab. XXXV, p , ' Mon. 

 Cretaceous Reptiles' ; and it was also articulated, as in the neck, by a tubercle, to the 

 extremity of the diapophysis. In a certain number of the anterior thoracic vertebrae, the 

 neck of the rib is- co-extensive with the diapophysis, and is sometimes six or seven 

 inches in length ; afterwards the neck of the rib begins to shorten, and the head to 

 decrease in size, and to have its place of articulation brought progressively nearer to 

 the tubercle and to the end of the diapophysis, until it finally disappeared, and the 

 posterior ribs became appended to the ends of the diapophyses. 



In the Iguana, as in other Lizards, the ribs have but one mode of articulation, viz., 

 to a simple tubercle developed from the side of the centrum. 



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

 Collection (No. |^), is 46 inches in length, its proximal or vertebral end is represented 



