Besides this remarkably close agreement as regards position, Kufra presents several natural features which would tend to support the view that it may be the locality which Ptolemy meant by the “Tortoise Marshes.” Not only is Kufra an extensive tract of relatively low-lying ground with numerous lakes and salt-marshes of very considerable size, but it has distinctly the form of a valley. Hassanein Bey repeatedly refers to it as a valley in his description[43]; Mrs. Forbes also speaks of the “Wadi of Kufra,” and mentions that as a result of a ride westward from Taj the “wadi” was found to have no definite ending to the west.[44] What more natural, therefore, than that some ancient traveller should have imagined that Kufra was a series of marshes formed by a river coming from the south-west? And is it not possible that Ptolemy, in endeavouring to piece together the scraps of information he could get, may have mistakenly inferred that this river ultimately reached the Mediterranean, and also have confused the account of it with those of other streams further south, which may have been branches of the Niger? 11. _The Sand-Dunes._ The sand-dunes of the Libyan Desert have been the subject of careful studies by Mr. Beadnell and Mr. Harding King, and practically all that was known concerning them up to the outbreak of the Great War is contained in the excellent papers by them which were read and discussed at meetings of the Royal Geographical Society in 1910, 1916, and 1918.[45] With the commencement of the Senussi campaign in Egypt in 1915, the sand-dunes sprang into new and unexpected importance, from the fact that they formed one of the principal hindrances to the free movement of troops, and more especially of military motor transport, across the desert. Every line of dunes of any considerable extent had now to be carefully mapped and examined for possible car-passages through it; the result was to add greatly to our knowledge of the distribution and extent of these remarkable features of the Libyan Desert, especially in the northern parts, where many long lines of dunes were found of which the existence was previously unsuspected by geographers, though they are familiar landmarks to the Bedouin of the region, and all except the smallest bear Arabic names. After the cessation of hostilities, interest in the distribution of dunes was maintained, because of the increasing use of motor-cars in place of camels for transport across the desert; and this circumstance has recently led to light being thrown on the distribution of the dunes in those southern regions which lay outside the field of operations during the war.