Key to the Common Genera of Freshwater Diatoms
| 1a. |
Radially symmetrical in valve view (valves can appear
rectangular if in girdle view) | [CENTRALES]
2 |
| 1b. |
Not radially symmetrical in valve view |
[PENNALES] 11 |
Centrales
| 2a. |
Valves with many intercalary bands (barely distinguishable),
valves in girdle view appearing rectangular and flattened, weakly
silicified, normally planktonic | 10
|
| 2b. |
Valves appearing otherwise | 3
|
| 3a. | Valves with two large
ocelli (appearing like bubbles at the margin) | Pleurosira |
| 3b. | Valves
appearing otherwise | 4 |
| 4a. |
Valves usually appearing in side view (therefore appear
rectangular), cells usually forming chains of varying number |
5 |
| 4b. | Valves disc
to drum shaped, valve mantles and girdle mostly reduced, cells generally
single | 6 |
| 5a. |
Valve mantle and valve without detail, areolae on
the mantle just distinguishable with light microscope |
Melosira |
| 5b. | Valve
mantle with easily distinguishable areolae | Aulacoseira (part) |
| 6a. |
Valves with a sulcus (a distinct ring when focused
through) | Aulacoseira
(part) |
| 6b. | Valves without a sulcus |
7 |
| 7a. |
Valves without distinctive features |
Melosira |
| 7b. | Valves otherwise |
8 |
| 8a. |
Areolae arranged radially around the valve margin ,
usually not extending to the center of the valve, central area distinct |
Cyclotella
|
| 8b. |
Central area otherwise | 9
|
| 9a. | Areolae arranged in clear
radial lines extending to valve centre | Cyclostephanoid
group |
| |
(Stephanodiscus,
Cyclostephanos, Actinocyclus, Thallassiosira) |
| 9b. | Areolae
arranged otherwise | Actinocylcus
, Thalassiosira |
| 10a. | Valves
with one long bristle/seta at both ends | Urosolenia
|
| 10b. |
Valves with two long bristles/seta at both ends |
Acanthoceras |
Pennales
| 11a. |
Valves without raphes | 12
|
| 11b. |
Valves with raphes | 20
|
Araphidae
| 12a. |
Valves without striae | Diatoma
|
| 12b. |
Valves with striae | 13
|
| 13a. | Valves heteropolar
(may only be slightly so) | 14
|
| 13b. |
Valves isopolar | 15
|
| 14a. | Valves swollen
at both ends | Asterionella
|
| 14b. |
Valves swollen at head end only | Meridion
|
| 15a. | Valve swollen
centrally and at ends, with septa inside frustule |
Tabellaria
|
| 15b. |
Valve otherwise | 16
|
| 16a. | Clearly
punctate striae, distinct hyaline central area | Ctenophora |
| 16b. | Valve
otherwise | 17 |
| 17a. | Shortened striae (<half
of valve width) | 18 |
| 17b. | Valve
otherwise | 19 |
| 19a. | Valves >
20 um; if shorter then with clearly defined central area |
Fragilaria
/Synedra |
| 19b. | Valves shorter |
Staurosira group (part) |
| (it
is difficult to separate Fragilaria, Synedra and Staurosira 'group'
in cleared material) |
| 20a. |
Valves with only vestigal raphes at distal ends on
ventral side | [PSEUDORAPHIDAE]
Eunotia |
| 20b. | Valves with at least
one true raphe | 21 |
| 21a. | Valves with raphes on
one valve only | [MONORAPHIDAE]
22 |
| 21b. |
Valves with raphes on both valves | [BIRAPHIDAE]
24 |
Monoraphidae
| 22a. |
Valves bent in girdle view | 23
|
| 22b. |
Valves +/- flat so rarely seen in girdle view |
Cocconeis
|
| 23a. | Coarsely
punctate; brackish water forms | Achnanthes
|
| 23b. |
At most finely punctate; generally freshwater |
Achnanthidium
|
Biraphidae
| 24a. |
Valves bent in girdle view; heteropolar, vestigal
raphe on one valve | Rhoicosphenia
abbreviata |
| 24b. | Valves straight |
25 |
| 25a. |
Valves with heavy costae | 26
|
| 25b. |
Valves without costae | 31
|
| 26a. | Valves completely
crossed by costae | 27 |
| 26b. | Valves
not completely crossed by costae | 29
|
| 27a. | Valves dorsoventral |
28 |
| 27b. | Valves otherwise |
Denticula |
| 28a. | Raphe lying along ventral
side rising dorsally at centre | Epithemia |
| 28b. | Raphe
lying along dorsal side | Rhopalodia |
| 29a. | Valves saddle-shaped |
Campylodiscus |
| 29b. | Valves
otherwise | 30 |
| 30a. | Valves transapically
undulate | Cymatopleura |
| 30b. | Valves
otherwise | Surirella
|
| 31a. | Raphe
in a canal crossed by fibulae | 32
|
| 31b. |
Raphe appearing otherwise | 36
|
| 32a. | Raphe canal
spiralling down length of valve | Cylindrotheca |
| 32b. | Raphe
straight | 33 |
| 33a. | Raphe canal central |
Bacillaria |
| 33b. | Raphe
usually lateral | 34 |
| 34a. | Valve with longitudinal
hyaline fold | Tryblionella
|
| 34b. |
Valve appearing otherwise | 35
|
| 35a. | Raphe canals
on same side of each valve | Hantzschia
|
| 35b. |
Raphe canals on opposite side of each valve |
Nitzschia
|
| 36a. | Valve
heteropolar | Gomphonema
|
| 36b. |
Valve isopolar | 37
|
| 37a. | Valve dorsoventral |
38 |
| 38b. | Valve appearing
otherwise | 41 |
| 38a. | Ventral side with swollen
central area | Reimeria |
| 38b. | Ventral
side otherwise | 39 |
| 39a. | Valve mantle usually
much wider at dorsal side, often with many intercalary bands |
Amphora
|
| 39b. |
Dorsal side only slightly wider | 40
|
| 40a. | Raphe rising
from ends to valve centre; distal raphe ends deflected dorsally |
Cymbella
|
| 40b. |
Raphe +/- parallel with ventral side not rising to
centre; distal raphe ends deflected ventrally | Encyonema
|
| 41a. | Valve in
S shape | 42 |
| 41b. | Valve
appearing otherwise | 43 |
| 42a. | Transverse and longitudinal
striae cross at right angles | Gyrosigma
|
| 42b. |
Valve with diagonal striae | Pleurosigma
|
| 43a. | Valve apically
twisted with raphe along margins of 'wings' |
Entomoneis |
| 43b. | Valve
appearing otherwise | 44 |
| 44a. | Locules (chambers) on
valve margin usually evident (also wavy raphe and punctate striae) |
Mastogloia
|
| 44b. |
Locules never present | 45
|
| 45a. | Most valves
<20 um; isolated stigma in central area | Luticola
|
| 45b. |
Stigma never present | 46
|
| 46a. | Raphe in
heavy rib, ending in an 'arrow' | 47
|
| 46b. |
Axial area may be wide but not with rib |
48 |
| 48a. | Striae interrupted forming
longitudinal lines | |
| 48b. | Striae
complete without longitudinal lines | 53
|
| 49a. | Longitudinal
lines hyaline, centrally constricted | Fallacia
|
| 49b. |
Characteristic untrue | 50
|
| 50a. | Raphe lying
in thickened axial area | Diploneis |
| 50b. | Characteristic
untrue | 51 |
| 51a. | Striae punctate and incomplete
forming several wavy longitudinal lines | Anomoneis
|
| 51b. |
Striae appearing otherwise | 52
|
| 52a. | Striae generally
clearly punctate and proximal raphe ends usually deflected in opposite
directions | Nedium
|
| 52b. |
Striae appearing otherwise | Caloneis
|
| 53a. | Valve with
clear central area extending to margin | 54
|
| 53b. |
Valve appearing otherwise | 56
|
| 54a. | Striae punctate,
sometimes not obviously so | 55
|
| 54b. |
Striae otherwise | Pinnularia&
Caloneis (part) |
| 55a. |
Cells rounded often appearing on their side |
Staurophora |
| 55b. | Valve
face relatively flat | Stauroneis
|
| 56a. | One valve
with craticulum, usually with parallel striae extending to axial
area | Craticula
[KEY] |
| 56b. |
Valve appearing otherwise | 57
|
| 57a. | Valve apices
hyaline, extending to valve margin | Sellophora
|
| 57b. |
Valve appearing otherwise | 58
|
| 58a. | Striae alveolate |
Pinnularia
& Caloneis (part) |
| 58b. | Valve appearing
otherwise | Navicula
[KEY] |
| For further information
or comments on the generic key, please contact Dr
Peter Gell |
| Created
by Jason Sonneman, last modified 13/07/99 |
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![[PSEUDORAPHIDAE] Eunotia](../eunser1.jpg){kind=link}
