In instituting a comparison between the maxillary organs of the Iguanodon, and those of the existing herbivorous lizards, we are at once struck with their remarkable deviation from all known types in the class of reptiles. In the _Amblyrhynchi_ (of the Galapagos Islands), the most exclusively vegetable feeders of the Saurian order, the alveolar process beset with teeth is continued round the front of the mouth: the junction of the two rami of the lower jaw at the symphysis presenting no edentulous interval whatever, the lips not being more produced than in other reptiles; but this creature only bruises its food; it cannot grind or masticate it. In fact, the edentulous, expanded, scoop-shaped, procumbent symphysis of the lower jaw of the Iguanodon, has no parallel among either recent or fossil reptiles; and we seek in vain for organs at all analogous, except among the herbivorous mammalia. The nearest approach is to be found in certain _Edentata_; as for example in the _Cholæpus didactylus_, or Two-toed Sloth, in which the anterior part of the lower jaw is destitute of teeth, and much prolonged. The correspondence is still closer in the extinct gigantic _Mylodons_, in which the symphysis resembles the blade of a turf-spade, and has no traces of incisor sockets; and were not this part of the jaw elevated vertically in front, and the two sides confluent, it would present the very counterpart of that of the Iguanodon. The great number and size of the vascular foramina distributed along the outer side of the dentary bone in the Wealden reptile, and the magnitude of the anterior outlets which gave exit to the vessels and nerves that supplied the front of the mouth, indicate the great development of the integuments and soft parts with which the lower jaw was invested. The sharp ridge bordering the deep groove of the symphysis, in which there are also several foramina, evidently gave attachment to the muscles and integuments of the under lip; and there are strong reasons for supposing that the latter was greatly produced, and capable of being protruded and retracted so as to constitute, in conjunction with a long extensile tongue, a suitable instrument for seizing and cropping leaves and branches, which, from the construction of the teeth, we may infer was the food of the Iguanodon.