It is towards this low-lying zone that all the great valleys of the hydrographic system of the Western Sahara seem to converge. It is to be presumed that, such being the conditions, the tracing of a hypsometric curve of 250 or 260 metres of altitude (that is to say, slightly superior to that of the actual Chad) would fix the limits, in the region of the Chad, the Lowlands of the Chad, and Borkou, of the ancient Central African lake zone, the existence of which is proved by the agreement of the geological, topographical, ichthyological, malacological, and other observations made in these regions in the course of the last twenty years. Are we to see in the remains of this former Caspian of the Sahara the Chelonide marshes of the geographers of the ancient world? To do so would not be altogether unreasonable if it be taken into account that, so far as I am aware, there is not to be found in the south-west of the Lybian desert any other low-lying region combining conditions so favourable to the existence of a vast zone of lake or marsh. Again, if we bear in mind certain local traditions declaring that towards the beginning of the nineteenth century native navigators were able to go in boats from the Chad to the Lowlands of the Chad by the Bahr el Ghazal (an assertion that the present appearance of Lake Kirri, recently dried up, makes sufficiently probable), one may conclude that until the early centuries of the Christian era this low-lying and now completely waterless region of the lowlands of the Chad may have been a great zone of lakes and marshes dotted with sandy or rocky archipelagoes. Other facts may equally be noted in corroboration of this hypothesis. Firstly, the numerous layers of shells of river molluscs and the large quantity of fish-bones to be met with there: among the latter a fragment of a skull and vertebræ examined by M. J. Pellegrin, which he thought were to be attributed to a Nile perch (_Lates Niloticus_, L.) of about 6 or 7 feet in length (in the _Comptes Rendus de l’Académie des Sciences_, tome 168, No. 19, p. 963. Séance de 12 May 1919); and the discovery of an elephant skeleton in a region where neither grass nor water is any longer to be found. Attention might also be drawn to the rock-drawings of Yarda, where hippopotami are represented among horses, camels, dogs, and ostriches; or to the numerous ruins of settled villages found all up and down, especially where the Bahr el Ghazal falls into the Djourab. Lastly, it may be mentioned that on the platform of certain rocks in Borkou may be found great cemeteries that a native chief attributes to a completely vanished race of “black Christians.” But our researches revealed to us no trace or vestige of Christian religion, perhaps because we could not devote enough time to them.