Environmental Engineering Reference
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(b) Cellsnotasabove..............
133
can cause blockage of filters. Bacillario-
phyta. Plate XIX. Fig. 4.58.
133
(132) (a) Valves large (35-130 μm) and
more heavily silicified. Many radially
arranged ribs and grooves on valve sur-
face originating from a clear central
area.......................
Ellerbeckia
Cyclostephanus
shows features that are
intermediatebetween
Stephanodiscus
and
Cyclotella.
134
(130) (a) Valve face with two or more circular
processes (ocelli) on the surface - clearly
visible as obvious large structures com-
pared with normal punctae. . ........
135
Ellerbeckia
cells are large diameter cylin-
ders sometimes joined in short chains.
Numerous small disc-shaped chloroplasts.
Clear differentiation between the mark-
ings on the valve face and the mantle,
which has a distinctive hatched pattern.
Previously included in
Melosira.
Found in
damp habitats including sandy and other
sediments. Only in plankton if sediment
stirred up. Bacillariophyta. Plates XX and
XXI.
(b) No obvious large circular structures
onthevalvesurface................
136
135
(134) (a) Cells widely elliptical or subcircular
in valve view. Two large ocelli on valve
surface which is also decorated with lines
around the margin and radiating from the
central area . ..................
Auliscus
Auliscus
cells are frequently attached to
sand grains, stones or rocks in inshore and
estuarine habitats. Although bottom living
they may be found in the plankton. Cells
40-80 μm in diameter. The shape of the
cells and the large ocelli are characteristic
of the species. Bacillariophyta. Plate XIX.
Fig. 4.59.
(b) Cells usually solitary but sometimes
forming loose chains. Valve surface with
radiating rows of punctae (single towards
the centre but often double or more
towards the edge) separated by clearly
defined ribs or clear areas. Central area
punctate but rows not as clearly defined.
Valve surface concentrically undulate
.......................
Stephanodiscus
(b) Cells circular in valve view. More than
2 marginal processes. Valve surface flat at
centre but raised as it goes towards the
marginal processes - which may have a
furrow running back towards the centre.
Valvesurfaceareolate.....
Aulacodiscus
Cells of
Stephanodiscus
are disc-shaped
and slightly barrel-shaped in girdle view.
The valve face is undulate with the cen-
tre either raised or lower than the mar-
gin. The valve margin has a ring of short
spines and in some small species long
delicate chitin threads that allow cells
to link together to form loose chains.
Numerous discoid chloroplasts sometimes
appearing to lie around the margin of the
cell. Common in freshwaters, especially
eutrophic ones, planktonic.
Stephanodis-
cus
has been reported (Palmer, 1962) as
producing geranium and fishy odours and
tastesindrinkingwaters.Highpopulations
A marine and brackish water species asso-
ciated with sediments where it can be very
common. The processes on the valve sur-
face are variable in number but normally
range between 3 and 8. Cell diameter
60-200 μm. Bacillariophyta. Plate XIX.
Fig. 4.60.
136
(134) (a) Valve surface apparently almost
devoid of markings and with large
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