186
THE ESSEX NATURALIST.
the extent of forming a right angle with the main axis of the
frustule (Fig. 6). I have also seen a case in which the little
knob of the slender part became temporarily stuck in some way
to the substratum while the diatom continued on its course so
that the terminal part assumed the form of a hook (Fig. 7).
When the little knob became released the terminal part straight-
ened itself immediately, showing that it was elastic as well as
flexible.
With regard to the possible special mode of division of
Cylindrotheca there is at present, so far as I know, no direct evi-
dence. But as the usual method of division in a diatom is for
the two valves to separate in a manner resembling the separation
of the two parts of a pill-box, after each part has developed a
new bottom part within itself, it is evident that the process cannot
happen in that simple way in a spirally twisted form. The
problem therefore remains, how does Cylindrotheca manage to
separate its two halves seeing that they are twisted about three
times around one another ?
Lastly a word or two about the occurrence of Cylindrotheca
generally and in Epping Forest in particular. There seems
little doubt that it is typically a brackish water form for it is more
often found in such water and reaches its largest size therein.
I have seen living specimens in brackish water from Littlehampton,
Shoreham and Appledore, and preserved specimens from the
Thames at Waterloo Bridge. Nevertheless it has also been
recorded many times from perfectly fresh water. On the whole
it is considered rare, only a few specimens being taken as a rule,
but it has been taken in considerable numbers occasionally.
It is probably cosmopolitan in its distribution, at any rate I have
a slide of an almost pure gathering labelled as coming from New
Zealand.
The record of its occurrence in Epping Forest is peculiar.
Some ten years ago I found a few specimens, mixed up with many
other species, in the little runnel by the side of the road through
Fairmead Bottom. I tried repeatedly to find it again in the same
and other localities in the district, but without success and I was
beginning to doubt whether it could be regarded as a permanent
element in the algal flora of the Forest. Quite recently, however,
when examining samples of water from the little pools which
have formed in most of the "bomb craters" scattered about
the Forest I was surprised to find a few specimens, mostly alive
and active, in no less than five such pools. Presumably, therefore,
Cylindrotheca has been present somewhere in the waters of the
district all the time, for we can scarcely imagine that it was
introduced into the craters by the bombs themselves. Anyway
it has now reached some additional habitats in these newly formed