ON M@RISIA LYONSI. S57 
Cordylophora and the Polyzoon, first established them- 
selves in this brackish lake. 
The course of events in Pleistocene times is at present 
rather obscure; it is thought, however, that this large pre- 
historic lake became disconnected both from the sea and 
from the Nile fjord, and that the greater part of it gradually 
evaporated, leaving probably only a small lake on the site of 
the present Fayim depression, the latter already separated 
from the Nile fjord by a rocky ridge. 
In later Pleistocene times, probably in consequence of 
slight elevation, the fjord became gradually silted up, and 
the early Nile cut itself a channel through the lacustrine 
beds. 
The Nile at this period Beadnell (16) supposes to have 
been flowing about 20 metres higher than at present; in 
some way or other its waters must have broken through the 
dividing ridge and the Faytim depression again became con- 
verted into a large lake. 
When this occurred there must have necessarily been a 
mingling of. the fauna of the old brackish lake and that of 
the early Nile; most probably the greater part of the former 
was exterminated by the influx of fresh water, Mcerisia, 
Cordylophora, and the ctenostomatous Polyzoon being 
among the few survivors. 
This new lake was the Lake Meceris of the ancients, and 
must have occupied an enormous area; Beadnell has estimated 
the latter to have been about 2250 square kilometres, or 
about ten times the area of the present Lake Qurun. The 
small size of the lake at the present day is due partly to 
evaporation, but chiefly to the reclamation of the land carried 
out during historic times by the different rulers of Egypt since 
the twelth dynasty. 
