Dr. H. Woodward—Supposed Pholas-borings, Fayim. 401 
with hard siliceous stems,' the final decay of whose upright stems have 
left the hollow cylindrical cavities which suggested to my friend, 
Mr. Beadnell, the resemblance to the crypts of boring Mollusca. 
Dr. C. W. Andrews, F.R.S., who has paid several visits to the 
Faytim in search of fossil mammalia, informs me that the tall Arundo- 
like reeds always grow in large clumps along the margin of the lake, 
and that the huge concretions, or globular masses of sandstone— 
honeycombed with vertical borings—represent in all probability the 
actual bases where these reeds formerly grew. 
He suggests that the accumulation which had been originally 
formed in the interstices between and around their thickly growing 
reed-like stems, was largely composed of the fine grains of sand-dust 
blown by the prevailing winds, and which had since become solidified 
into a concretionary sandstone by the addition of calcareous and saline 
matter contributed by the waters of the lake itself. 
On this point Mr. Beadnell observes (p. 13, op. cit.): ‘‘ Although 
under the present desert conditions practically no material from the 
surrounding desert is washed into the lake, doubtless a considerable 
amount of fine dust and sand is carried into it by the wind, especially 
during the violent sandstorms which occur frequently in the locality. 
The high cliffs which bound the northern shore of the lake, throughout 
a portion of its length, probably have the effect of checking the 
velocity of both north and south winds, thus causing a considerable 
amount of sand, which would otherwise be carried across, to be 
dropped on its surface” (p. 14). 
The occurrence of these globular masses of concretionary sandstone, 
perforated by countless numbers of vertical holes, at different levels, 
both at and above the present margin of the lake, is readily explained 
as being due to the gradual shrinkage of the level of the lake,? which . 
is constantly going on from evaporation and supposed underground 
outlets, and this, together with the quantity of water absorbed by 
the large area (1800 kilometres) under cultivation in the Faytm, is 
probably in excess of that received by the Bahr Yusef Canal, the 
natural inlet from the Nile. 
The vast proportion of perforations made by Saaicava and by 
LInthodomi generally, penetrate the rock in all directions, and offer 
no very close analogy to these usually straight, tubular, parallel, closely 
arranged hollows. 
But the crypts of Pholas erispata are at times more regular, often 
forming extensive colonies, and the animals, in burrowing, less seldom 
invade one another’s retreats. They commence as quite small crypts, 
but as the molluse increases in size and the perforation deepens, it 
expands its chamber /aterally, which, seen in vertical section, becomes 
somewhat flask-shaped with a slender neck. The lower end is 
rounded and has the largest diameter. Should the boring mollusc 
pass through the ledge and so expose the lower extremity of the 
1 See pl. xvi. View near the western end of the Birket el Qurin, also view of 
the north side looking west, pl. i, in Mr. Beadnell’s Memoir on the Topography and 
Geology of the Fayim Province of Egypt (Cairo, 1905). 
* Now occupying only 225 square kilometres, the whole area of the Fayim 
depression, much of which was once a lake, covering about 12,000 square kilometres. 
DECADE V.—VOL. VII.—NO. IX. 26 
