A Key to the More Frequently Occurring Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 205

taste to drinking waters (Palmer, 1962).
Pyrrophyta. Plate XVII (showing Cer-
atium hirundinella and C. cornutum ).
Figs. 1.3, 1.9, 1.10, 2.7, and 4.56.

two flagella. Cell wall thin and some-
times lightly ornamented with about 20
plates. Numerous yellow-brown chloro-
plasts. Widespread in ponds and swamps.
Also found in saline situations. Epitheca
(above the central groove) and hypotheca
(below the central groove) of similar size.
Pyrrophyta. Plate XVII.

(b) Cells otherwise shaped ......... 124

124 (123) (a) Cell wall thin and difficult to see
(except in empty cells) or completely
absent............................ 125

Woloszynskia is a genus with epicone and
hypocone of nearly equal size and with
up to 100 delicate thin plates over the
cell surface. An eyespot may be present.
Plate XVII.

(b) Cell wall thick, cellulose plates cov-
ering the cell easily seen. Transverse fur-
row encircling the central area of the
cell........................ Peridinium

126 (106) (a) Cells isolated or in groups, wall
siliceous decorated with grooves or dots
(punctae) which form a definite pattern
or other definite markings on the sur-
face. Storage products (mainly lipids and
droplets) may be visible within the cell.
Chloroplasts one to many, yellow-green
to golden-brown in colour .......... 127

Peridinium cells are normally ovoid in
outline and free swimming. The cell is
covered with angular plates arranged in
a specific order with an epitheca and
hypotheca of approximately equal size.
There are numerous brown chloroplasts.
A widespread genus found in a range of
freshwater habitats. Reported to impart
both taste and odour to drinking waters
(Palmer, 1962). Pyrrophyta. Plate XVII.
Figs. 1.8 and 4.57.

(b) Cell wall not made of silica or deco-
rated with patterned dots or bars. No con-
spicuous lipid droplets present ...... 204

127 (126) (a) Cellscircularinoutlineinvalveview. 1
Decorations usually arranged in radial
rows or radial segments, sometimes quite
faint; or in the form of large processes
(ocelli, see Glossary). 2 Cells often solitary
but can occur in loose chains. ....... 128

125 (124) (a) Cells without plates on walls
........................ Gymnodinium

Cells (7-80 μm wide, 8-118 μm long)
approximately oval but sometimes flat-
tened dorsiventrally. A median transverse
groove is present. Epicone (above median
groove) and hypocone (below median
groove)aboutequalinsize.Thecellwallis
thin and smooth. Pyrrophyta. Plate XVII.

1 NB. Campylodiscus may appear circu-
lar in one view but is saddle-shaped in
another. This is a distorted form of a pen-
nate diatom, not a centric one, so will key
out through section (b). (b) Cells with thin walls with plates but

thesearedelicateanddificulttoseeunless
cellisempty............. Glenodinium

2 NB. Plate XVIII shows some typical
examples of the markings on the valve
faces of centric diatoms to indicate the
types of patterns seen, not all of which,
however, are included - see also Plates
XIX and XXI.

Cells (13-48 μm wide, 25-50 μm long)
approximately oval in shape, some-
times dorsiventrally flattened with groove
around the mid-region bearing one of the