M ore than forty years have passed since the late indefatigable Gideon Mantell's discovery of a tooth first brought to light the former existence of the Iguanodon. Since then the remains of this great Dinosaur have been plentifully obtained from the Wealden districts in Kent, Sussex, and the Isle of Wight; and of most segments of its skeleton we now possess a better knowledge than we have of the bony frame of any of its larger contemporaries. It is of the heads of these Dinosaurs that our knowledge continues to this day very defective. Tooth-bearing portions of the mandibles and maxillæ of Iguanodon Mantelli are now to be seen in the principal public and in some private collections; but of crania proper which can with much probability be referred to Iguanodon , the only remains known to me are a mutilated base, with side-walls, and several fragments of the vault and sides of the skull in the Rev. W. Fox's collection, obtained near Brixton, Isle of Wight, and the skull which, several years ago, I brought under the notice of this Society, and of which a figure is given in vol. xxvii. pl. xi. of the Quarterly Journal. It came from the Wealden clay at Brooke. The articular element of the mandible is one of the missing parts which has escaped recognition (no mandible yet figured, or, so far as I know, in any collection, possesses i t ); and I am happy to have now an opportunity, through the courtesy of