As all who have paid attention to the Dinosauria are aware, our knowledge of the structure of the skull in these extinct reptiles is very defective. This is particularly true of Megalosaurus, of which, up to the present time, only a portion of the lower jaw has been known. I am therefore very glad to be enabled, by the kindness of Mr. Abbay, the possessor of the magnificent left upper jaw of Megalosaurus now exhibited, to make a definite addition to our means of reconstructing that monstrous Saurian.