Abstract When in the year 1825 I had the honour to lay before the Royal Society a notice on the teeth of an herbivorous reptile found in the limestone of Tilgate Forest, in Sussex, I entertained the hope and expectation that the discovery of the jaws, or a portion of the jaw with the teeth attached, would reward my labours, and enable me either to confirm or modify the inferences I had ventured to deduce from an examination of the teeth alone. And I was encouraged in this anticipation by the remarks of Baron Cuvier, who, in the correspondence upon this subject which which he honoured me, thus expressed himself:-"N'aurions-nous pas ici les dents d'un animal nouveau, d'un reptile herbivore? Le temps confirmera ou infirmera cette idée, puis -qu'il est impossible qu'on ne trouve pas un jour une partie du swuelette, réunie à des portions de mâchoire portant des dents." But after unremitting research, and the collection of several hundred teeth, and bones of various parts of the skeletons of many individuals, even including the tympanic bone with the auditor cells, I found myself, at the expiration of twelve years, without any additional information respecting the maxillary organs of this colossal reptile, no trace of the jaws having come under my observation. I have, however, at length had the good fortune to discover what appears to be a portion of the lower jaw of a young Iguanodon, in which the fangs of many teeth, and the position of the germs of several of the successional teeth are preserved.