ON MCRISIA LYONST. 373 
Meerisia, yet the salinity is known to have varied greatly 
even in historic times, and at the time when great cities 
such as Dimeh were constructed on the north side of the 
lake there is little doubt that the Birket el Qurun contained 
fresh water. Even at the present day, although the amount 
of salt is sufficient to make the water unpalatable, an analysis 
showed that at the west end of the lake (where the concen- 
tration is greatest owing to the distance of the feeder canals) 
the total salts amounted to only 1:34 per cent., of which 
‘92 per cent. was sodium chloride.! 
At present, excluding Mcerisia, only three genera of 
medusz have been described from the freshwater systems of 
the different continents. These are: Limnocodium, 
originally described from the tanks in the Regent’s Park 
Botanical Gardens; Limnocnida, from Central Africa; and 
last the medusa of Microhydra recently found in North 
America, 
An admirable summary of the structure of these forms has 
recently appeared in the ‘ Quarterly Journal of Microscopical 
Science,’ vol. 50, to which I must refer for more detail (18). 
Other medusz have from time to time been recorded from 
lagoons in close proximity to the sea, but such forms, e.g. 
von Kennell’s Halmomises (14) from Trinidad, and 
Annandale’s Irene (15) from the Ganges Delta, cannot be 
regarded as belonging to the true freshwater fauna. 
As in the case of the above-mentioned meduse, the question 
arises: How did a marine organism lke Meerisia find its way 
into Lake Qurun ? 
Before attempting to solve this question a little more detail 
‘of the topography and geology of the Fayim must be given. 
Lake Qurun measures at the present day approximately 
~ twenty-five miles by six ; it lies in a depression on the border 
of the Libyan desert, its waters being nearly 140 feet below 
the level of the sea. 
The Fayim depression is separated from the Nile Valley 
1 Quoted by Beadnell from a note by Schweinfurth in Willcock’s ‘ Egyptian 
Irrigation.’ 
VOL. 52, PART 8.—NEW SERIES. 29 
