WEALDEN FORMATIONS. 9 



order than the Iguana and the rest of the Lizard-tribe. The two costal articulations, 

 viz., for the head and tubercle of the rib on the anterior dorsal vertebrae,* and the 

 corresponding modifications of some of the ribs themselves,! indicated the great 

 extinct herbivorous reptile of the Wealden to have enjoyed a double circulation, aided 

 by a four-chambered heart as perfect in structure as that of the modern Crocodilia.\ 

 The peculiar expansion and complexity of the neural arch, so superior to the structure 

 of that part in the Crocodile, and so distinct from the Ophidian modification of the same 

 part in the Iguana, § pointed to the former existence of a primary group of the class 

 Meptilia, superior to and distinct from both the crocodiles and lizards of the present 

 period. But the confirmation of the ideas and the resolution of the questions thus 

 suggested depended on, or at least rendered very desirable, the detection of 

 corresponding modifications of other parts of the vertebral column, and especially of 

 that part which more immediately transferred the weight of the hinder parts of the 

 trunk and the tail upon the — for a reptile — enormously developed hind limbs. 



No sacrum of any recent or fossil cold-blooded animal had at that time been 

 recognised as including more vertebras than the typical number — two — in the reptilian 

 class ; and no single vertebra, or set of vertebrae, had then been referred to the 

 sacrum of the Iguanodon. The Mantellian Collection in the British Museum, according 

 to its catalogue, in the hand-writing of its founder, contained no sacral vertebrae. I 

 suspected, indeed, at an early period of my investigations of our Fossil Reptilia, some 

 detached centrums, or vertebral bodies, of a young Iguanodontoid Saurian, in that 

 Collection, No. ^w, e , g, to belong, from certain characters hereafter to be noticed, 

 to the sacral region of the spine ; but the proof, from better preserved specimens, 

 was still wanting. 



Every collection, public and private, to which I could command access, was 

 ransacked for a specimen that might agree with the suspected characters of the great 

 desideratum towards completing the vertebral anatomy of the Iguanodon. Some years 

 passed away, leaving fruitless this research; until, in 1840, in the course of a systematic 

 examination of the private collections in this metropolis, and whilst engaged in the 

 comparison of the reptilian fossils in the well-stored museum of J. Devonshire Saull, 

 Esq., F.G.S., in Aldersgate Street, City, my attention was arrested by a bulky and 

 weighty mass of anchylosed and fossilized vertebrae, with a long and rather flattened 

 bone attached to one side, the examination of which left a conviction of their agreement 

 in general form and characters with the supposed sacral vertebrae and iliac bones of 

 the Iguanodon in the British Museum. The following is the description of this sacrum 

 of the Iguanodon given in the part of my 'Report,' published in 1841, where it is 



* 'Report,' 1841, pp. 126, 127. f lb., p. 133. 



% 'Report, Brit. Foss. Rep.,' 1841, p. 203. § 'Hist. Brit. Foss. Reptiles,' 4to, 1850, p. 136. 



2 



