The geological evidence for the existence of a great freshwater lake in Pleistocene and prehistoric times is afforded by the well-marked lacustrine clays and sands which are found over such a large area of the northern and western deserts of the Fayûm; the great extent of this lake will be seen by an examination of the accompanying maps. Its area must have been about 2250 square kilometres or about ten times the size of the modern Birket el Qurûn. The western limit may even have been further west than shown on fig. 10.,[91] as some of the desert in the neighbourhood of Gar el Gehannem is very low-lying; or there may have existed subsidiary lakes in that direction. The upper limit of these ancient lacustrine clays is between 22 and 23 metres above sea-level, which exactly agrees with the figure adduced by Sir Hanbury Brown as the height of the more modern Lake Moeris from the evidence of levels. That Lake Moeris was simply the older prehistoric lake placed under artificial control admits of no doubt; the difficult question being as to when the lake first came into existence in prehistoric times. The clays abound in freshwater shells and semi-fossil fish-bones of exactly the same species of fish (siluroid, etc) as still inhabit the Birket el Qurûn; probably some are even hardly different specifically from the Middle Eocene forms of the district. In addition remains of large animals are common, and include _Hippopotamus_, _Elephas_, _Bubalis_, sheep or goat, and _Canis_, with crocodiles and turtles, etc.[92] [Illustration: PLATE XV. ISOLATED SAND-DUNE NEAR GAR EL GEHANNEM.] Martens[93] has described the following species of mollusca from Schweinfurth’s collections:— _Unio abyssinicus_, Mart. _U. Schweinfurthi_, Mart. _Corbicula fluminalis_, var. _consobrina_ Caill. _Neritina nilotica_, Reev. _Valvata nilotica_, Jick. _Cleopatra pirothi_, Jick. _C. pirothi_, var., _unicarinata_, Mart. _Bithynia_ aff. _Boissieri_, Charp. _Melania tuberculata_, Müll. _Limnaea natalensis_, Krauss. _L. mœris_, Mart. _L. palustris_, Müll. _Planorbis subangulata_, Phil. Blanckenhorn has pointed out[94] that this fauna is of special interest and differs from all fossil and living faunas in Egypt. It might be compared with the _Melanopsis_-fauna of the Nile Valley if the exceptional _Limnaea_ were replaced by _Melanopsis_ or _Paludina_. Its _Unio Schweinfurthi_ recalls the youngest alluvial deposits of the Nile Valley, 2nd Cataract, Kom Ombo and Silsila; at these places, however, the beds containing the species in question are at least 20 metres above mean water level of the present day.