A Key to the More Frequently Occurring Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 223

egg-shaped or narrowing towards the cen-
tre (slipper-shaped). Iso- or heteropolar.

(b) Cells more mildly crescent-shaped
with both margins showing more gentle
convexcurves..................... 161

.................................. 155

159 (158) (a) Cells strongly lunate (moon-shaped)
with parallel sides, raphe difficult to see
other than in girdle view and then only
towards the end of the valve. Valve apices
strongly arcuate. Conspicuous transverse
ridges on the valve face extending into
short spines at the cell margin. Two plate-
likechloroplasts............. Semiorbis

155 (154) (a) Cells with permanent rib-like mark-
ings around the margin . ............ 156

(b) Cellsnotasabove.............. 157

156 (155) (a) Cells isopolar. .... Surirella group a

(b) Cells heteropolar. . Surirella group b

Semiorbis cells (20-40 μm long, 3.0-
5.5 μm wide) have rounded apices, a
reduced raphe (not conspicuous) and can
occur singly or in short filament-like
groups. Usually found in upland areas,
sometimes in coloured waters, and is
not particularly common. Bacillariophyta.
Plate XXIII.

Surirella cells (20-400 μm long, 10-
150 μm wide) have rounded to slightly
pointed ends that may be iso- or heteropo-
lar. The raphe runs around the margin
of the cell on a wing supported by ribs
which give the characteristic markings on
the frustule surface. There are two large
lobed and plate-like chloroplasts but as
each lies beneath the valve face only one
is visible in valve view. Widespread on
sediments and rocks. Some species found
in brackish and marine habitats. Surirella
group a includes S. angusta (isopolar),
while group b includes S. tenera and S.
robusta (heteropolar, with widely rounded
apicesatoneendandmorenarrow,slightly
more pointed apices at the other). Bacillar-
iophyta. Plate XXII.

(b) Cells not lunate. ............... 160

160 (159) (a) Cells with swollen area on concave or
ventralside........................ 163

(b) Cellsnotasabove.............. 161

161 (160) (a) Cells gently curved into a crescent or
nearly straight and with bluntly rounded
apices. Raphe visible towards the poles of
thecell....................... Eunotia

NB. Species of Surirella that are not so
widely oval but are more linear to ellipti-
cal, may be keyed out elsewhere. Eunotia cells (17-220 μm long, 1.5-

7.0 μm wide) not as strongly curved
as Semiorbis . Sometimes with a slightly
undulateconvexmarginbutoftenasmooth
curve. Apices bluntly rounded or capi-
tate/reflexed. Raphe only visible towards
the cell apices where the polar nodules are
obvious. Two chloroplasts lying side by
side (often only visible as two in valve
view). Lives attached to other surfaces,
particularly plants, by means of mucilage
pads. Can be single or in ribbon-like
strands. Found in slow flowing or standing

157 (155) (a) Cells crescent-shaped (like the seg-
ment of an orange), with one edge strongly
convex and the other either concave or
nearly straight, isopolar ............ 158

(b) Cells not crescent-shaped or as above
.................................. 165

158 (157) (a) Cells strongly crescent-shaped or dis-
tinctly curved in valve view (the more
commonview).................... 159