ANALYSIS OF THE GENERA.
1. Frustules n u d e ................................................................................................ 2
„ enveloped in gelatine or enclosed in membranaceous tubes 46
2. Frustules not forming a filament........................................................ 3
„ forming a filament or c h a in ...................................................... 28
3. Frustules free or adherent .................................................................... 4
„ attached by a cushion or stipes ............................................ 23
4. Frustules furnished with canaliculi................................................... 5
„ not so furnished ......................................................................... 7
5. Valves arcuate or inflated ........................................................ E p ith em ia.
„ saddleshaped........................................................... Campylodiscus.
„ elliptical, ovate or linear ................................................................ g
6. Valves concave, aim marginal ................................................. S u r i r e l l a .
„ plane, aim submarginal ........................................ T ryblionella.
7. Valves triangular .................................................................... T riceratium.
„ orbicular ...................................................................................... g
„ not triangular nor orbicular..................................... 12
8. Valves s tria te d ............................................................................. C y c l o t e l l a .
„ c e llu la r.............................................................................................. g
9. Valves p la n e .................................................................................. jq
„ und u la ted ..................................................................... Actinocyclus.
10. Discs of Valves furnished with processes............................. E u p o d isc u s.
» ), not so fu rn ish e d ............................................................... j j
1 1 . Valves with a central pseudo-nodule .......................... A r a c iin o id is c u s .
„ without a central nodule........................................ Coscinodiscus.
12. Valves arcuate...................................................................................... E u n o t ia .
„ not arcuate .......................................................... 2^
13. Valves cyinbifonn.................................................
„ elbptical, oval or linear........................................................... j 5
14. Nodule subcentral ........................................................ Cymbella.
,. margmal ............................................................................... Amphora.
1,5. Frustules a d h e r e n t......................................................................... Cocconeis.
„ free .................................................................................................
16. Valves transversely undulated ....................................... Cymatopleura.
„ without undulations......................................................................... 17
17. Valves with longitudinal ridges ............................................................... 18
,, plane or convex................................. 20
18. Valves with a central ridge or k e e l ........................................................... 19
„ with two or more ridges ......................................... Amph ipleu ra .
19. Valves elliptical, deeply constricted ................................... A mphiprora .
„ much elongated ; constriction, if any, s l ig h t N itzschia.
20. Valves sigmoid ......................................................................... P leurosigma.
„ s tr a ig h t................................................................................................. 21
21. Valves with central nodule ........................................................................ 22
„ with central nodule dilated into a Stauros ............ Stauroneis.
22. Valves s tria te d .................................................................................. Navícula.
„ costate ............................................................................. P innularia.
23. Frustule linear or much elongated ............................................. Synedra.
„ elliptical or cym b ifo rm .............................................................. 24
„ cuneate ....................................................................................... ■ 25
24. Valves having terminal and subcentral nodules ................. Cocconema.
„ without nodules............................................................... Doryphora.
25. Valves with a nodule in the centre and at each en d .. . . Gomphonema.
„ without nodules.................................................................................. 26
26. Stipes obsolete or very s h o r t................................................. P odosphenia.
„ elongated ........................................................................................... 27
27. Stipes filiform......................................................................... R hip idophora .
„ in c ra ssa te......................................................................... L icmophora.
28. Filament compressed, continuous............................................................... 29
,, a zigzag chain ............................................................................. 39
„ cylindrical...................................................................................... 4d
29. Filament spiral ............................................................................ Me r id io n .
„ coiled............................................................................. Bacillaria.
,, s tra ig h t........................................................................................... 3b
30. Frustule with internal s ep ta ........................................................................ 37
„ without septa.................................................................................. 31
31. Valves with central and terminal n o d u le s ............................. Diadesmis.
,, without nodules.................................................................................. 32
32. Frustule curved on Front View............................................ Achnanthes.
„ oblong on Front View ................................................................ 33
d . .