WEALDEN FORMATIONS. 21 



deduce such conclusions as to the nature of the soft parts that covered the lower jaw, 

 as the characters of that bone may legitimately sustain. 



The subject of Tab. XI, XII, is the dentary piece of the right moiety or ramus of 

 the lower jaw. It is chiefly remarkable for the straightness and parallelism of the 

 upper, a b, and lower, c d, borders, for the portion which the dentary piece contributes 

 to the suddenly rising coronoid process, /, and for the abrupt slope downwards, at an 

 angle of about 45°, of the short, edentulous, compressed anterior part of the bone, 6, 

 to the shorter symphysis, <?, d, fig. 1 ; which latter part of the bone projects a little below 

 the lower level of the ramus. The exterior surface of the ramus is, vertically, a little 

 concave where it forms the alveolar wall, and then becomes moderately convex to the 

 thick and rounded lower border, Tab. XII, fig. 4. A few foramina, g g, fig. 1, open at 

 irregular intervals, in a longitudinal series, upon the concave part of the outer surface 

 of the ramus, from 5 to 6 lines below the alveolar border ; and a few foramina occur 

 parallel with the sloping border, b c, at a similar distance from it. The general surface 

 of the bone on the outer side of the jaw is smooth, but becomes more irregular near 

 the symphysis ; it presents several lines of fracture, but rises to form the coronoid 

 process,/, without any trace of the suture which separates the coronoid from the 

 dentary piece in the jaw of the Iguana. The relative position of that suture, indeed, 

 to the termination of the dental series, in the Iguana, is such that the suture could not 

 be repeated in the Iguanodon, so as to include the coronoid process, because the dental 

 series is continued backward along the inner side of the base of that process. In the 

 form, extent, and direction of the coronoid process, it closely resembles that of the 

 Iguanodon, at least as regards so much of the process as is contributed in fossil by the 

 dentary piece. If its extent were added to by a true coronoid element articulating 

 with it behind, it would resemble the broader coronoid of the Cyclodus and Varanus. 

 The presence of the process, though formed in an unusual way in the Iguanodon, gives 

 the jaw a lacertine character, and makes it differ in a striking degree from that of the 

 Crocodilia. It remains to be seen, however, in more complete specimens, whether the 

 coronoid piece actually contributes any share to the process, or whether it be restricted, 

 as in the Crocodilian reptiles, to the inner surface of the ramus, bounding the fore part 

 of the wide entry to the mandibular canal. 



The inner side of the dentary element of the mandible of the Iguanodon, Tab. XI, 

 fig. 2, displays, as in the Lacertia generally, the alveolar recesses, and such traces 

 of the teeth themselves as may have been preserved, which in the present case are 

 limited to a few more or less advanced germs of successional teeth. This aspect of 

 the jaw shows that the dentition of the great extinct herbivorous reptile was of the 

 " pleurodont"* type, as in the Iguana and many other modern lacertine genera. 



* 'Odontography,' p. 240 ; the term signifies the attachment of the teeth to one side or parapet of an 

 open alveolar groove. 



