WEALDEN FORMATIONS. 23 



edentulous character of that part are peculiarities in which the Iguanodon differs 

 from all known modern reptiles. 



Another character by which the Iguanodon differs from modern lizards, and 

 especially from the Iguana, is the contour of the alveolar plate viewed from above, as 

 in fig. 3, Tab. XII ; it is thus seen to describe a gentle but graceful sigmoid curve, 

 convex inwards at its hinder two thirds, straight in the rest of its extent, or slightly 

 concave inwards, as continued by the edentulous symphysis. In the Iguana the 

 hinder four fifths of the alveolar plate is straight ; it bends inwards to the symphysis 

 of the jaw at its anterior part. The form of the thick rounded lower border of the jaw 

 of the Iguanodon is shown at fig. 4, pi. xi. 



In the Iguana the mandibular groove runs nearer the base of the alveolar plate 

 than the lower border of the ramus, and stops short before it has reached the middle 

 of the dental series : in the Varanus the same groove extends from the anterior 

 termination of the splenial piece along the lower border of the ramus as far as the 

 symphysis ; in regard to this groove, therefore, the Iguanodon resembles the Varanus 

 and also the Cyclodus more than it does the Iguana. In the Crocodiles one sees only 

 an oblong foramen at the fore-end of the splenial element. The inner plate or wall of 

 the dentary bone in the Iguanodon bifurcates behind, as in most reptiles, where it 

 articulates with the splenial, angular, and coronoid elements ; the upper branch is 

 shown at h, the lower one at c, fig. 2, Tab. X. What may have been the length of the 

 entire jaw, as completed by the splenial, angular, surangular, and articular elements, 

 must remain conjectural, until either this part of the mandible, or an entire upper jaw 

 with the tympanic part of the same cranium, may be discovered. 



In the Iguana the dentary element forms about three fifths of the length of the 

 lower jaw ; in the Cyclodus it forms rather more than half, in the Varanus a little less 

 than half of the lower jaw ; in the Crocodile it forms more than two thirds the length 

 of the jaw. 



As the dentary piece in the Iguanodon itself contributes to the formation of the 

 coronoid process, it is probable that the entire jaw may more nearly resemble the 

 Crocodilian than the Lacertian type in the proportion of the ramus formed by the 

 dentary element. . 



The length of the corresponding element of probably a mature Iguanodon, figured 

 in the 'Philosophical Transactions' for 1848, and now in the British Museum, is 21 

 inches ; its vertical diameter, in a straight line, where the alveolar wall is best pre- 

 served, is 3 inches, 7 lines, so that it is relatively deeper than in the younger Iguanodon, 

 and this probably in reference to a deeper implantation of the large teeth of mature 

 age, and to the greater strength of the jaw required for the more vigorous mastication 

 at that period of life. The coronoid process, Tab. XIII, /, being a part of the dentary 

 bone, has also been preserved with the rest of that element in Capt. Brickenden's 

 specimen, and shows the same abrupt curve upwards. The nervo- vascular foramina are 



