More 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 it); and I am happy to have now an opportunity, through the courtesy of Mr. Fox, of bringing before the Society a form of articular bone to which, from intrinsic characters, and also from its gisement, the Iguanodon appears to me to have a paramount claim.
After several years of fruitless search in every accessible collection for the Iguanodon's os articulare, Mr. Pox, in 1869, submitted to me a bone the skeletal place of which he had not been able to decide; and indeed, apart from the rest of the mandible, its identification by one who had not specially studied anatomy offered considerable difficulty. There could not, however, be any doubt that the bone in question was the detached os articulare of a very large reptilian mandible. Mr. Pox had long possessed three, which, as he afterwards told me, he had obtained from the same localities that had yielded him magnificent mandibular rami of Mantell's Iguanodon. I suggested at the time to their fortunate possessor that he should publish an account of these valuable specimens; but it appeared to him wiser to wait for additional evidence confirmatory of their really belonging to the Iguanodon. Since then he has obtained two others, making five in all, of which four belong to right rami; and in May of last year he proposed to me to communicate to the Society an account of them.
Taking the least imperfect specimen as the type (in all the ante-
