Several charts of Mars are in existence, but as the same nomenclature has not been employed in all of them, some confusion has arisen in regard to the names given to the most remarkable features of the planet's surface. In order to give clearness to the subject, it will be necessary here to give a brief description of the principal markings represented on Plate VIII. In this the nomenclature will be employed which has been adopted by the English observers in the fine chart of Mr. Nath. Green. The large dark spot represented on the left-hand side of the plate is called De La Rue Ocean. The dark oval spot, isolated in the vicinity of the centre of the disk, is called Terby Sea; while the dark, irregular form on the right, near the border, represents the western extremity of Maraldi Sea. The dusky spots of Mars seem to be permanent, and to form a part of the general surface of the planet. That several among them, at least, are permanent, is proved by the fact that they have been observed in the same position, and with the same general form, for over two centuries. Yet, if we are to depend upon the drawings made fifty years ago by Beer and Maedler, it would seem that the permanency of some of them does not exist, since a very large spot represented by these astronomers on their chart of Mars is not visible now. This object, which, on their map, has its middle at 270°, should be precisely under the prominent dark oval spot called Terby Sea, seen near the centre of the picture, and would extend down almost as far as the northern limb. This can hardly be attributed to an error of observation, since these observers were both careful, and had great experience in this class of work. It is a very singular fact that, at the very same place where Maedler represented the spot in question, I found a conspicuous dark mark on December 16, 1881, which was certainly not visible in 1877, during one of the most favorable oppositions which can ever occur. The object, which is still visible (Feb., 1882), consists of an isolated spot situated a little to the north of Terby Sea. During the memorable opposition of 1877, I investigated thoroughly the markings of Mars, and made over 200 drawings of its disk, 32 of which represent the Terby Sea; but this isolated spot was not to be seen, unless it be identified with the faint mark, represented on the plate, which occupied its place. There cannot be the slightest doubt that a change has occurred at that place. Changes in the markings have also been suspected on the other hemisphere of the planet.