236 THE ESSEX NATURALIST.
VOLVOCALES.
This group is nearly always well represented in the nanno-
plankton of fresh waters and that of the bomb-crater pools was
no exception. The most frequently occurring genus, as might
be expected, was Chlamydomonas, especially very small forms
of from 21/2 to 10u. Unfortunately it is very difficult to identify
many of the forms seen. The following representatives of the
order were noted :—Carteria globosa (1)2, C. spp. (4), Chlamydo-
monas ? minima (1), C. ? minutissima (1), C. spp. (12), Chloro-
gonium elongatum (2), Dysmorphococcus sp. (1), Gonium sodale (1),
Heteromastix angulata (1), Hyalogonium acus (1), Phacotus sp. (1),
Polytoma ? obtusum (1), Scourfieldia complanata (3) (Fig. 6),
Spennatozopsis exultans (1) (Fig. 1),
Chlorococcales.
This group also is generally well represented in the nanno-
plankton and the following forms were recorded from the
bomb-crater pools :—Ankistrodesmus falcatus, usually the very
long variety acicularis, but sometimes a twisted variety (6),
Chlorella sp. (4) (Fig. 5), Crucigenia tetrapedia (1), Kirchneriella
lunaris (1), C. ? contorta (1), Scenedesmus quadricauda (1), S.
obliquus (1), Selenastrum ? gracile (1), S. minutum (1).
CONJUGALES.
The majority of forms belonging to this group are too large to
be reckoned among the nannoplankton, though there are a few
species of Desmids which can pass easily through the finest nets.
They do not, however, appear to occur very frequently and in
the bomb-crater pools only the following have been seen :—
Closterium sp., a small needle-shaped form C ceratium (2),
Staurastrum muticum 20u. (1).
Chrysophyceae.
Many members of this class are typical nannoplankton
organisms and one or more forms are to be expected in most
samples of water. The forms seen in the bomb-crater pools
were :—Chromulina sp. (1), Chrysococcus rufescens (4) (Fig. 3),
C. spp. (4), Dinobryon sociale var. americanum (1), Mallomonas
sp. (1), Sphaleromantis ? ochracea (1), Stenocalyx sp. (2), Synura
uvella, small colonies (2).
Bacillariophyceae.
Small forms of Diatoms are often found in the nannoplankton,
but it is not as a rule possible to determine the species owing to
difficulty of seeing the characteristic markings in the living
2 The figures in brackets after the specific names refer to the number of pools in which
the organism has been found out of a total of 27. The specific names in the algal groups are,
as far as possible, in accordance with the "Susswasserflora" (Pascher et. al., 1913-1927).