WEALDEN FORMATIONS. 13 



foramen. The coalesced extremities of the fourth and fifth sacral ribs have been 

 broken away on the left side. These coalesced extremities form a continuous tract of 

 bone, pi V — pi 4', Tab. Ill, with a flattened outer surface, slightly concave lengthwise, 

 to which the iliac bone is attached, and has remained attached probably through partial 

 confluence on the right side of the present specimen. The organic architecture of this 

 part of the vertebral column of the ancient gigantic reptile cannot be sufficiently 

 admired in reference to the due strength of the part thereby attained. 



The pressure transmitted by the thigh bones upon the iliac bones is resisted, and 

 is transferred by the strong vertical buttresses, formed by the modified and coalesced 

 pleur- and di- apophyses, upon the bodies and neural arches of the sacral vertebrae ; 

 but, by the altered relative position of the neural arches and pleurapophyses, the 

 weight transmitted by one pair of buttresses does not bear exclusively upon a single 

 vertebral centrum, but is divided equally between two centrums. In the young and 

 perhaps more active Iguanodon, prior to the general anchylosis that afterwards 

 pervades this complex mass, the further advantage of a certain elastic yielding of the 

 parts must have been gained, by the implantation of the piers of the neural arch, and 

 the heads of the short, buttress-like ribs, upon or over the joints between the vertebral 

 bodies. A like advantage is gained by the same modification, in regard to the position 

 of the neural arches and ribs, in the vertebrae of the carapace of the Chelonian reptiles, 

 and in the sacral vertebrae of the Ostrich ; the structure of the latter is interestingly 

 analogous to that of the same part of the spine in the extinct Iguanodon.* 



A considerable proportion of the right iliac bone remains attached to the sacrum of 

 the Iguanodon above described. It is a strong, elongated, subtriedral bone, firmly 

 adherent by an inner flattened surface to the confluent expanded ends of the five sacral 

 ribs, pi \—pl 5. The outer surface is divided into two facets by a strong longitudinal 

 ridge, for the attachment of some of the powerful muscles of the hind limb, and a 

 second short, oblique, almost vertical, ridge, divides the bone into an anterior and a 

 posterior portion. The anterior portion is again subdivided into a thick, strong, 

 acetabular portion 62—62, forming the upper part of the cavity for the hip-joint, and a 

 more slender portion extending forwards as far as the anchylosed lumbar vertebrae, and 

 terminating in an obtuse point, 62'. The hinder portion of the ilium, 62", is extended 

 backwards beyond the surface of attachment to the sacral ribs, and probably terminated 

 freely, as in most Lacertian reptiles ; but the extremity of this part has been broken 

 off.f The chord of the acetabular arc or concavity measures 8 inches. The major 

 part of the acetabulum was contributed, as in most modern lizards, by the ischium and 

 pubis. Upon the whole, the structure of the ilium accords more with the Lacertian 

 than the Crocodilian type of the bone. 



* See my 'Archetype of the Vertebrate Skeleton,' 8vo, 1848, p. 159,%. 27. 



f In the Paper, 'Phil. Trans.,' 1849, by the authors who anticipated my illustrations of Mr. Saull's 

 pelvis of the Iguanodon, I am stated, or at least the author of a ' Report on British Reptiles' is charged, 

 p. 299, with having mistaken the anterior for the posterior part of the ilium. A reference to p. 135 of the 



