Among the fresh-water snakes, Dr. Günther tells us of one, _Hydrinus_, which is semi-pelagic, and which indulges in little excursions down the rivers to exchange greetings with his marine relatives, some of whom, on their part, occasionally go a certain distance up the rivers. Again, among the sea snakes is one who rambles for change of air or diversity of diet over the fields and far away. In him, Dr. Günther describes one of those many transitions found in every class and order throughout nature. _Platurus_ is his name; he has the ventral scales of land snakes to enable him to wander over the salt water marshes which he loves. His nostrils are on the side of his head instead of on the top, and his head shields differ from those of all his relatives. His venom fangs are small, and his tail is not prehensile, presenting the united characters of fresh and salt water and land snakes. Thus we have links between sea and land snakes, between fresh water and salt, and between these latter and fishes, for in many instances the affinities are so close that naturalists have doubted in which class to place them. When that remarkable animal, the _Lepidosiren_, which Darwin calls a living fossil, was first brought from Africa some thirty years ago, it was found to present so many characteristics in common with both reptiles and fishes, that it was for some time a mooted question in which class to place it. In appearance it more resembles the former, with its four curious filamentary limbs, which Owen considers ‘the beginnings of organs which attain full functional development in the higher vertebrates.’ The same high authority has decided that the only character which absolutely distinguishes fishes from reptiles, so closely are some of them allied, is whether or not there is an open passage from the nostrils to the mouth; and the ‘Lepidosiren’ is now known as ‘the mud-fish of the Gambia,’ the ichthyic characters predominating. Sea snakes were not unknown to the ancients. Aristotle mentions them (Taylor’s Translation, 1812, Book ii. vol. 6), ‘Of sanguineous animals, however, there remains the genus of serpents. But they partake of the nature both of terrestrial and aquatic animals. For most of them are terrestrial, and not a few are aquatic, and which live in potable water. There are also marine serpents similar in form to the terrestrial genus, except that their head more resembles that of a conger. There are, however, many genera of marine serpents, and they are an all-various colour; but they are not generated in very deep places.’