234 THE ESSEX NATURALIST.
THE BOMB-CRATER POOLS.
The craters in the great majority of cases form completely
isolated basins. A few, to be sure, happen to be situated in
communication with a pond or in the course of a small stream,
but these have been ignored in the present investigation. Owing
to the fact that the Epping Forest district is based on the geologi-
cal formation known as the London Clay and that this clay is
actually at the surface, except for some isolated patches of river,
glacial and other gravels, the craters nearly always retain the
rain water which falls on them. The pools thus formed vary
considerably in size (5 to 20 feet or more), depth and character
of their water, depending largely of course upon the season and
weather and also upon the area, depth and situation of the craters
in which they occur. As regards situation, the majority of
craters are actually among the trees of the forest and they there-
fore receive considerable quantities of dead leaves, but there are
also a good many in quite open spaces. Sometimes the water of
the pools is rather turbid and yellowish owing to the washing
down of the clay from the sides of the craters, but often it is
practically clear and then it may appear colourless or blackish,
bluish, greenish or brownish. In some pools the water appears
quite free from any visible plant growth, but in others there has
developed a certain amount of filamentous algae and in some
cases a few higher plants.
It follows from what has been said (1) that the pools in the
craters dealt with are entirely dependent for their existence on
rainwater, (2) that they are self-contained units and (3) that they
started de novo in newly excavated hollows. It was these facts
about the pools which suggested that a study of the nanno-
plankton in them might lead to interesting results in connection
with the powers of dispersal of such organisms and their
dependence upon the particular character of the water. There
was also the possibility of new or little known species making
their appearance under conditions so essentially different from
those normal in the district.
NANNOPLANKTON OF THE BOMB-CRATER POOLS.
The groups of organisms represented in the nannoplankton
have been alluded to in the introductory remarks and they may
now be taken in detail in the order already given.
Bacteria.
It goes without saying that all the pools produced Bacteria
of one sort or another, but they were not as a rule very much in