‘So, then, a mobile tooth was already known to science.’ Of _Bucephali viridis_, Dr. Andrew Smith describes the ‘posterior or _mobile_ and grooved teeth of the maxilla.’ He says: ‘Some are placed for immediate use, the rest are recumbent between those and the inner portion of the spongy sheath which envelops them; anterior teeth fixed.’ He considered these back teeth not poisonous, but only for holding or preventing the escape of food. ‘They may convey an acrid saliva.’ Still we are not informed _how_ the teeth move.[113] These snakes—the Bucephali—like the far-famed horse of Alexander the Great, owe their name to their large, ox-shaped head. They are the ‘Boomslange’ or tree snake of the Dutch settlers, and are by some ophiologists included among the _Dendrophidæ_, or true tree snakes, as they live in trees; but Dr. Andrew Smith considers that their teeth sufficiently separate them from these. That there is something exceedingly interesting to study out in the _Xenodon_ family cannot be doubted. ‘The transition begun in the Bucephali,’ says Owen,[114] ‘is completed in the poisonous serpents,’ but where the virulent character of the saliva begins it is hard to say. Despairing of any distinct comprehension of a jaw-bone which permits of moveable back teeth, the last resource was to hunt up a skeleton. At the Museum of the Royal College of Surgeons none was to be found; but through the kindness of the officials at the British Museum, one was at length unearthed from the subterranean labyrinths of untold treasures there. It was the skull of _X. gigas_, the largest of the family, and a splendid specimen for examination. There were two large posterior fangs on each side. On one side were two or three more large reserve fangs—a cluster of them. All were recumbent. They were all much larger than that of _X. rhabdocephalus_, those in reserve varying in size relatively to their development and position. In this specimen there were also two double rows of palate teeth, and an abundant but most disorderly row of simple teeth in the lower jaw, with some reserve ones packed closely on the inner side below the row in use. They exactly illustrated the words of Nicholson and others, ‘the crop of young teeth everywhere working their way into the intervals of the old ones.’ In the skulls of _Liophis meremii_ and _Liophis cobella_, of which Dr. Wucherer says, ‘Dentition similar to Xenodon,’ the former had teeth gradually increasing a trifle posteriorly, but nothing like fangs. _L. cobella_ had a very long jaw of fifteen or sixteen teeth, but no fangs. On a second occasion I made a dental examination of the living _Xenodon_ in order to be fully convinced of the nature of its back teeth, and in both instances the fangs were depressed until the snake was provoked into displaying them. It exhibited no spitefulness or attempt to bite, and in both cases folded back its fangs the moment my finger was removed, as if glad that the ceremony was over.