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Class Bacillariophyceae Haeckel 1878 emend. D.G. Mann in Round et al. (1990)
The Diatoms: Biology and Morphology of the Genera. Cambridge.

Cells elongate and often symmetrical about at least two planes, one of which is the pervalvar plane; heterovalvar cells also occur. Stria pattern bilateral, orientated about a longitudinal sternum, as in Fragilariophyceae; raphe present (sometimes subtended internally by siliceous bridges), either within the longitudinal rib or to one side of it (Eunotia). Costae and septa may be present. Rimoportules absent (except Eunotia and related genera). Usually one, two or four plastids per cell, occupying characteristic positions in relation to the raphes; rarely many small, discoid plastids. Chain-like, zig-zag or stellate colonies rarely formed, but epiphytic species often form stalks for attachment.

Genera in Bacillariophyceae


Achnanthes Bory (1822)

Cells are heterovalvar and are flexed in girdle view. They are solitary (occasionally forming short chains) attaching directly to a surface by one valve face, or by a mucliage pad or stalk at one valve apex. A single plate-like plastid, lies under the rapheless valve and extends under the girdle. Valves are linear to elliptical or bluntly lanceolate, with rounded or, in a few cases, protracted apices. The raphe valve is on the convex side of the shallow 'v' caused by the flexing of the girdle. The central area is variable, and is sometimes replaced by a thickened stauros. The rapheless valve is sometimes very similar to the raphe valve, except for the absence of a raphe, but in other cases, it is quite different, typically with coarser striation and differences in the size of the sternum. Both valves must be observed to ensure accurate identification.


Achnanthidium Kütz. (1844)

Cells are linear, lanceolate or elliptical when seen in valve view, and are bent about the median transapical plane to form a shallow "v" (sometimes with reflexed apices) when seen in girdle view. They are heterovalvar, with the central raphe thickening usually distinct on the concave valve. The cells are attached to the substrate by a short mucilage stalk secreted from one end of the raphe valve and often remain joined after division to form short chains. There is a single plastid, lying against the one side of the girdle and extending beneath the rapheless (convex) valve. The raphe valve has a narrow, linear axial area and fine, radiate striae. There is sometimes a distinct central area but in other cases the striae at the centre are just more widely spaced. The fine structure of the raphe is usually indistinct when viewed with the LM, but neither central nor polar endings are obviously thickened. The rapheless valve is similar, except that there is no raphe and the central area is small or absent. Striae on the rapheless valve vary from slightly radiate to transverse and parallel.


Amphipleura F. T. Kützing (1844)
Die kieselschaligen Bacillarien oder Diatomeen
Valves linear-lanceolate to slightly rhombic with acutely, rounded ends. Very distinctive due to short raphe-slits restricted to poles.


Amphora Ehrenberg in Kützing (1844)

Valves lunate, but entire cells bi-convex due to mantle on dorsal margin being much deeper than that on ventral margin. Raphe usually towards ventral margin and often sinuous to strongly arcuate. Dorsal margin sometimes with hyaline areas


Aneumastus Mann & Stickle ex Round et al. (1990)
Diat. Biol. Morphol. Gen.:663
Cells broadly lanceolate in valve view with rostrate or subrostrate apices, narrowly rectangular in girdle view. Live cells with two lobed, chloroplasts, each with a central pyrenoid, arranged fore and aft in the cell, usually solitary. Valves coarsely striate, striae sometimes becoming biseriate near the margins. Raphe slits central, undulate, continuing to valve apices. Several girdle bands with rows of pores.


Anomoeoneis Pfitzer (1871)

Valves lanceolate to elliptical-lanceolate with broadly rounded to capitate ends. Striae distinctly punctate. Usually with hyaline areas on either side of the axial area. Distal raphe ends clearly deflected to one side.


Bacillaria J.F. Gmelin (1791)
Systema Naturae, vol. II, ed. 13, part 6, p. 3903
Bacillaria is most easily recognized when alive, because of the unique motile colonies it produces, in which the cells, though linked to each other via a ridge and groove arrangement of their raphe systems, are able to move over each other, so that the colony can alternate between a Fragilaria-like chain and an extended, fibre-like configuration, in which each cell is connected to its neighbour only by its tip. Frustules isopolar. Cells seen in valve or girdle view. Valves shallow, always lying in valve view, bilaterally symmetrical, linear or linear-lanceolate, with pointed, rounded, cuneate to slightly rostrate poles. Striae transverse, appearing as simple lines. Raphe system fibulate, central. The raphe itself is invisible in the light microscope, but its presence can be detected because of the fibulae, which are rectangular (long axis transapical) and are often connected by delicate longitudinal ridges linking their bases. Central raphe endings are absent. There are two chloroplasts per cell, one towards each pole. The central gap between them is where the nucleus lies.


Brachysira Kützing (1836)

Valves linear to linear-lanceolate to rhombic with rounded to protracted ends. Striae finely punctate, and appearing to form longitudinal undulations. Raphe straight and axial area narrow.


Caloneis Cleve (1894)

Valves linear to broadly lanceolate sometimes centrally inflated or undulate. Ends cuneate, rounded, rostrate to sub-capitate. Axial area often broad and centrally inflated and slightly asymmetrical or "scruffy". Raphe +/- straight with ends hooked to one side. Striae visible and usually course, some species lacking striae in central area.


Campylodiscus Ehrenberg ex Kützing (1844)
F.T. Kützing (1844). Die kieselschaligen Bacillarien oder Diatomeen. Nordhausen. p. 59
Solitary. Frustules isopolar. Mantles and girdle shallow, so that cells are most often seen in valve view. Valves circular but often appearing elliptical or dorsiventral, because the frustules are bent in girdle view, each valve being saddle-shaped. The saddle-shape, circular outline and apparently pennate structure (i.e. a median rib or hyaline area subtending lateral striations) make this genus unmistakable. Runnng around the whole perimeter of the valve is a raphe system that opens inwardly into a tubular canal ('canal-raphe') raised on a ridge or keel. The canal is separated from the remainder of the cell interior by siliceous bridges (fibulae) and often also by infoldings of the valve face and mantle, which, in some species where the keel is especially high, fuse to create extracellular windows (fenestrae) beneath the raphe. In the light microscope, the raphe itself is almost impossible to detect: its presence must be deduced from the presence of fibulae or fenestrae. The valve pores are usually very small and the striae that they form are also inconspicuous. Much more obvious are rib-like thickenings and wavy infoldings of the valve, the areas of pores and striae that these surround, and the fibulae. The raphe system in fact contains two raphe slits, each running around half of the valve perimeter. There are therefore two sets of 'central' raphe endings, one at each side of the valve. Within a frustule, the raphe endings of the two valves are offset by 90°. There is probably only one chloroplast per cell, consisting of two large plates, often lobed at their margins, connected by a narrow isthmus. The plates lie against the valves and fill all except the cell periphery when seen in valve view. However, further study is needed. The median rib or hyaline area is only superficially similar to the sternum or raphe-sternum of other pennate diatoms. Valve development in Surirellales, as in other raphid diatoms except Eunotiales, occurs from the raphe system outwards.


Cavinula Mann & Stickle ex Round et al. (1990)
Diat. Biol. Morphol. Gen.: 665
Cells solitary with one or two chloroplasts, usually H-shaped in girdle view, comprising lobed plates under the valves, connected by a bridge containing the pyrenoid. Valves linear-lanceolate or rhombic-lanceolate, to more or less elliptical; girdle narrowly rectangular. Raphe slits central, stopping short of the apices, deflected in opposite directions at the apices. Striae uniseriate, fine, pores sometimes elongated along the striae. Girdle composed of narrow bands with transverse rows of pores.


Cocconeis Ehrenb. (1837)

Cells are solitary and heterovalvar with a concave (sometimes only slightly) raphe valve and a rapheless valve that is convex, often very strongly so. They live attached to substrates by mucilage produced by the raphe valve. There is a single C-shaped plastid, which is sometimes lobed and which contains one to several elongate pyreneoids. Valves vary from almost circular in outline to elliptical with rounded, or slightly pointed, ends. The raphe valve has fine, radiate striae with a narrow, linear axial area and a small central area. There is often a hyaline area at or close to the margin of the valve. The rapheless valve has coarser striae, which are often obviously punctate, a linear or lanceolate axial area and no central area. The convex curvature of the rapheless valve is often apparent under the light microscope. There are a few narrow girdle bands, none of which possess pores.


Craticula Grunow (1868)
Grunow, 1868. Reise Fregatta "Novara" Bot. 1 (i): 20
Cells usually lanceolate to linear lanceolate in valve view with subrostrate to rostrate apices, narrowly rectangular in girdle view. Live cells solitary, with two plate-like chloroplasts lying along the girdle, and extending slightly under the valve face. Often with 2 conspicuous droplets between the central cytoplasmic bridge and the cell apices. Valves with central raphe slits, continuing over the apices, slightly expanded at centre. Striae often more or less parallel, uniseriate with fine regular pores. Valves may appear as if transected also by longitudinal striae. Few girdle bands, each with a single row of pores.


Cymatopleura W. Smith (1851)
W. Smith (1851). Notes on the Diatomaceae, with description of British species included in the genera Campylodiscus, Surirella, and Cymatopleura, Ann. Mag. Nat. Hist. ser. 2. 7: 1-14, p. 12 (nom. cons.).
Cells solitary. Frustules isopolar. Mantles and girdle shallow, so that cells are most often seen in valve view. Valves elliptical to elongate, sometimes constricted at the centre. Raphe system running around the whole of the valve perimeter, raised very slightly above the valve face (never as high as in some Campylodiscus and Surirella spp). The raphe system is detectable by the presence of fibulae and the edges of the subraphe canal, which can appear as a submarginal line running close to and parallel to the perimeter. Highly characteristics is the prominently undulate valve face, which is homogeneously structured, containing very fine striae that bend in various directions according to their positions relative to the undulations. There is no axial structure, but a line is detectable, formed where opposing sets of striae meet during valve development. There can also be slight irregular thickenings of the valve face, giving a speckled appearance. Short ribs may be present towards the periphery of the valve face: these are continuous with bar like fibulae subtending the raphe. Alternatively, the fibulae may be circular or flared rectangular structures. Chloroplast(s) appearing to fill the whole cell in valve view, appearing as two large plates with lobed margins connected by an isthmus at one end of the cell, or as highly dissected reticulum.


Cymbella Agardh (1830)

Valves asymmetrical around apical axis and symmetrical to the transapical axis. Cells showing slight to pronounced dorsiventrallity. Ends rounded to subcapitate. Raphe central or slightly ventral, +/- straight to sinuous. Stigmata lacking or on the ventral side of the central area. Terminal raphe ends straight or deflected dorsally. Single H-shaped chloroplast in most species (C. lanceolata is the exception) with a central pyrenoid located towards the dorsal margin.


Cymbellonitzschia Hustedt in A. Schmidt (1924)
F. Hustedt in A. Schmidt, Atlas Diat. pl. 352, figs. 12, 13. (1924). R. Reisland, Leipzig
Cells solitary or forming short chains linked via their valve faces (but with no siliceous linking structures). Frustules isopolar. Cells seen in valve or girdle view. Valves bilaterally asymmetrical (dorsiventral), with a straight ventral margin and a curved dorsal margin. Transverse striae visible, regularly spaced, but pores within them invisible in the light microscope. A fibulate raphe system runs along the ventral or dorsal side of the flat valve face. The raphe itself is invisible in the light microscope, its presence being detected by the presence of fibulae, which are small square or rectangular structures. Two chloroplasts per cell, one towards each pole. The central gap between them is where the nucleus lies.


Denticula Kützing (1844)
F.T. Kützing (1844). Die kieselschaligen Bacillarien oder Diatomeen. Nordhausen. p. 43
Cells solitary or occasionally forming very short chains linked via their valve faces (but with no siliceous linking structures). Frustules isopolar. Cells seen in valve or girdle view. Valves bilaterally symmetrical, linear, lanceolate or elliptical, with bluntly rounded to slightly protracted poles. Valve traversed by prominent partitions, appearing as bars ('costae') in the light microscope. These partitions are extensions of the fibulae beneath the raphe. Between the partitions are striae, which can sometimes be resolved as single or double lines of pores. A fibulate raphe system is present, either subcentrally or near the margin of the valve face. The raphe itself is invisible in the light microscope, but its presence can be detected because of the presence beneath the raphe of a subraphe canal, whose edges appear as longitudinal lines crossing the partitions. Between the fibulae/partitions, faint but distinct elliptical apertures can be detected, which are the openings between the subraphe canal and the cell lumen; the aperture is formed by flaring of the fibula bases. There may or may not be central raphe endings; if present, they are usually detectable in LM as a slight thickening of the wall of the subraphe canal at the centre. There are two chloroplasts per cell, one towards each pole. The central gap between them is where the nucleus lies. The cytoplasm is sometimes highly granular and 'sparkly'.


Diadesmis Kützing (1844)
Kies. Bacill. Diat.: 109
Cells small, more or less linear, solitary or forming band-like colonies. Live cells with a simple or slightly lobed chloroplast extending from the girdle under one valve, occasionally under both valves. Valves linear or linear lanceolate with bluntly rounded apices, valve faces sharply differentiated from mantles, sometimes with a row of peripheral spines at the junction. Girdle almost as deep as valves wide. Raphe central , often terminating before valve apices, in a broad sternum, sometimes filled in. Striae uniseriate, pores sometimes elongated along stria. Girdle comprising few to many bands with one or two rows of round pores. Spines may interdigitate and link adjacent valves.


Diatomella Greville (1855)
Ann. Mag. Nat. Hist. Ser. 2, 15: 259
Cells forming chains or zig-zag colonies. Valves linear to elliptical, with bluntly rounded apices. Girdle much deeper than valve is wide. Striae biseriate and alveolate, raphe central in a broad hyaline area, with quite widely separated central endings, polar endings extending over valve apices. Girdle bands without pores, but valvocopulae with septa that meet at two points in the cell lumen creating three circular openings.


Didymosphenia M. Schmidt in A Schmidt (1899)

Valves heteropolar, large and robust, symmetrical to apical axis and broadly capitate at both apices. Cells wedge-shaped in girdle view. Striae strongly punctate. Short terminal spines can sometimes be seen at the head pole. No pseudosepta. 2-5 stigmata occur on one side of the central area.


Diploneis Ehrenberg (1844)

Valves elliptical to panduriform with broadly rounded apices. Thickened canals are present on either side of the raphe slit and usually have a pattern of ornamentation that differs from the marginal striae (often seen as longitudinal lines either side of the axial area). Cells often robust in appearance.


Encyonema Kützing (1833)

Valves dorsiventral and symmetrical to the transapical axis. Dorsal margin normally highly arched, ventral margin more-or-less straight or slightly convex. Valve apices bluntly rounded to subcapitate. Stigmata absent or, if present, only on the dorsal side of the central area. Raphe more-or-less straight with central endings deflected dorsally and apical ends deflected ventrally. Single H-shaped chloroplast with a central pyrenoid on the ventral side of the cell.


Epithemia Kützing (1844)
F.T. Kützing (1844). Die kieselschaligen Bacillarien oder Diatomeen. Nordhausen. p. 33
Cells solitary or occasionally forming very short chains linked via their valve faces (but with no siliceous linking structures). Frustules isopolar. Cells when alive are more often seen in girdle view, but isolated valves lie in valve view. Valves and cells dorsiventral, with a straight or concave ventral margin and an arched dorsal margin; shape linear, linear-elliptical to lanceolate. Valve poles either bluntly rounded or broadly rostrate to capitate. Stria pores almost always easily visible in the light microscope; valve structure appearing like an irregular square-mesh sieve. The valve is traversed by prominent partitions, which appear in valve view as dark bars ('costae'). In girdle view, the partitions often seem to have expanded, bead-like ends, because they are capped by overlapping inward extensions of the girdle bands. The partitions span the raphe and act there as fibulae. Raphe system fibulate, biarcuate. Each branch of the raphe is marginal near the poles, but curves inwards to become subcentral, or even marginal on the opposite side of the valve, at the centre. The raphe itself is difficult to see in the light microscope, but its presence can be detected via the subraphe canal beneath the raphe, which appears as two closely spaced, parallel, longitudinal lines, which are continuous across the partitions. Between each pair of adjacent partitions are one or several elliptical apertures (faint but detectable in LM), which connect the subraphe canal and the cell lumen. One large chloroplast per cell, appressed to the ventral side of the cell and lobed at its edges where it extends into the spaces between the valve partitions. Careful observation of the central part of the cell, especially in girdle view, will reveal the presence of one to several ellipsoidal bodies with ± homogeneous, slightly darker contents, in addition to the nucleus (which is more difficult to detect). These are 'endosymbiotic cyanobacteria', found in all species of Epithemia and Rhopalodia. See comments under Rhopalodia for more information.


Eucocconeis P.T.Cleve (1895)

See description for E. flexella.


Eunotia Ehrenberg (1837)

Valves dorsiventral and symmetrical to the transapical axis. Dorsal margin convex, smooth or undulate, ventral margin straight or concave. Valve apices bluntly rounded to capitate. Striae extend across entire valve face. Raphe mainly on valve mantle and restricted to the poles. Raphe curved slightly or strongly onto the valve face at the apices. Terminal nodules usually conspicuous. Frustules box-like or rectangular in girdle view. Raphe often only visible in girdle view. Two elongate chloroplasts lie on the ventral side of the cell and extend onto the valve faces.


Fallacia Stickle & Mann ex Round et al. (1990)
Diat. Biol. Morphol. Gen.: 667
Cells solitary. Valves linear, lanceolate to elliptical, with bluntly rounded apices, girdle view narrowly rectangular. Striae uniseriate, but interrupted by lateral hyaline areas that connect with the central area. Raphe central in a narrow axial area, terminal fissures extending over valve apex, central fissures straight and expanded. Girdle bands without pores. Live cells with a single, H-shaped chloroplast with its narrow isthmus against the epivalve.


Frustulia Rabenhorst (1853)

Valves rhomboidal to linear-lanceolate. Apices bluntly rounded to (rarely) subcapitate. Raphe central and contained in a median rib extending most of the length of the valve. At the apices the raphe rib has the appearance of a pencil tip. Striae fine and arranged so as to produce a pattern of apical and transapical rows.


Gomphocymbella (Cleve) Hustedt (1930)

Valves transapically and apically asymmetric. Appearance very like that of Gomphomema but valves slightly dorsiventral. Both margins convex. Apices rostrate to subcapitate and +/- ventrally deflected. Raphe +/- central and slightly curved. Single stigma on dorsal side of central area.


Gomphonema Ehrenberg (1832)

Valves asymmetrical to transapical axis (heteropolar) and symmetrical to apical axis. Cells usually wedge-shaped in girdle view with pseudosepta visible. Apices rounded to capitate. Raphe often slightly sinuous. A single stigma (occasionally more) usually present on one side of the central area. Striae often coarse. There is a single H-shaped chloroplast with a central pyrenoid.


Gyrosigma Hassall (1845)
Hist. Brit. Freshw. Algae 1: 435.
Cells sigmoid in valve view, narrowly rectangular in girdle view, solitary. Live cells with two plate-like chloroplasts lying along each side of the girdle, sometimes lobed under the valve faces, with several rod shaped pyrenoids along their length. Valves sigmoid, raphe sigmoid, more or less central, apical fissures extending to the apices, central fissures usually deflected in opposite directions. Striae transverse and longitudinal. Girdle bands plain, without any pores.


Hantzschia Grunow (1877)
A. Grunow (1877). New diatoms from Honduras. Mon. Microsc. J. 18: 165-186. p. 174, nom cons.
Cells solitary Frustules isopolar but dorsiventral. Cells seen in valve or girdle view. Isolated valves almost always in valve view. Valves bilaterally asymmetrical (dorsiventral), with a slightly concave, straight or slightly convex ventral margin and a convex dorsal margin. Poles simply rounded, rostrate or capitate. Transverse striae visible, regularly spaced, uni- or (in some marine species) biseriate; stria pores within them sometimes visible in the light microscope. The fibulate raphe system can be biarcuate (marginal near the centre, ± central near the poles, or runs along the ventral edge of the valve face. In biarcuate species, the raphe itself can be seen with care in the light microscope, but where it runs along the ventral margin, its presence can be detected only by the presence of fibulae, which are rib-like, square or rectangular structures. Central raphe endings are usually present and are detectable in LM as a slight thickening of the wall of the subraphe canal at the centre. In a frustule, the raphe systems of both valves lie on the same side ('hantzschioid symmetry') Two chloroplasts per cell, one towards each pole. Each chloroplast usually consists of two plates, one appressed to either side of the girdle, connected by a narrow but robust bridge containing a pyrenoid. The central gap between the chloroplasts is where the nucleus lies.


Karayevia Round et L. Bukhtiyarova (1996)

Elliptic to lanceolate valves sometimes with drawn-out apices. Striation patterns differs between raphe and rapheless valves, with raphe valve having fine radial striae whilst rapheless valve has coarser, parallel striae. Axial area is narrow on both valves and the raphe is straight with polar endings turned to one side.


Kolbesia Round et L. Bukhtiyarova (1996)

Elliptic to elliptic-lanceolate valves with bluntly-rounded, rostrate ends. The striae are slightly radial on both valves and are each composed of a few elongate areolae. The axial area is ellipticdal or lanceolate on both valves, extending to about one third of the total breadth. The raphe is straight with a hooked external polar ending and expanded central ending.


Lemnicola Round et P.W. Basson (1997)


Luticola D.G. Mann ex Round et al. (1990)
Diat. Biol. Morphol. Gen.: 670
Cells solitary. Live cells with a single lobed chloroplast, lying with its centre against one side of the girdle and the lobes extending under each valve. Valves linear, lanceolate or elliptical with bluntly rounded, sometimes subcapitate to capitate apices. Raphe slit central, ending before the poles, with laterally deflected polar fissures. Central fissures also deflected to same side. Uniseriate striae comprised of distinct pores, often with a hyaline interruption around the valve margin. Isolated pore (stigma) present on one side of central area. Girdle narrowly rectangular, girdle bands with one or two rows of pores.


Mastogloia Thwaites ex W. Smith (1856)
W, Smith 1856, Syn. Brit. Diat. 2: 63.
Cells solitary or growing within mucilaginous masses. Live cells with two, lobed chloroplasts lying fore and aft in the cell, each with a central pyrenoid. Valves linear-lanceolate to elliptical, with slightly protracted, bluntly rounded apices. Raphe fissure central, undulate, extending to apices. Striae uniseriate or occasionally biseriate, transverse, comprised of coarse pores. Valvocopulae loculate (chambered), girdle bands perforated by one or two rows of pores.


Navicula Bory de St. Vincent (1822)
Dict. Class. Hist. Nat. 2: 128
Cells solitary, usually seen in valve view and often actively motile when live. Live cells with two plate-like chloroplasts lying along each side of girdle, each with a single rod-shaped pyrenoid along their length (only visible in girdle view). Valves variable in outline, usually linear-lanceolate to lanceolate, with variously shaped apices. Raphe central, fissures usually hooked over valve apices, slightly expanded at centre. Stria path variable, striae usually cross-lineate, i.e. containing linear pores at right angles to the direction of the stria. Girdle usually narrow, comprised of a few plain bands.


Neidium Pfitzer (1871)
Bot. Abh. Morphol. Physiol. 1: 39
Cells solitary. Live cells with four chloroplasts, lying alongside the girdle, one in each cell quadrant and containing a rounded pyrenoid. Valves often linear in their central region, with bluntly rounded, sometimes subrostrate or rostrate apices, girdle view rectangular, narrower than the valve width. Raphe slit central, in a narrow axial area, the central fissures usually deflected in opposite directions, the polar fissures forked. Striae uniseriate, comprised of fine rounded or elongate pores, more or less transverse, interrupted by longitudinal lines (canals) near the valve margins. Girdle bands plain, without pores.


Nitzschia Hassall (1845)
A.H. Hassall (1845). A history of British freshwater algae. S. Highley, N. Bailličre, Edinburgh, Paris, Leipzig. p. 435, nom. cons.
Cells solitary or, more rarely, forming stellate or chain-like colonies, or living in mucilage tubes. Frustules isopolar. Cells and valves seen in valve or girdle view, depending.on species Valves structurally asymmetrical, because the raphe system is almost always placed off-centre. The valve outline may appear asymmetrical and dorsiventral, but is usually symmetrical if seen in the correct orientation (i.e. with the valve margins in a plane perpendicular to the line of sight). But as usually seen, valves are symmetrical or asymmetrical about the longitudinal axis. They are usually highly elongate, may or may not be constricted or nicked centrally, and have rounded, rostrate or capitate poles. Transverse striae sometimes visible but often delicate, very fine or unresolvable in LM. Raphe system fibulate, usually appearing to run along one edge of the valve but subcentral in a few species; except in a few cases (e.g. N. filiformis, N. obtusa), the raphe maintains the same position on the valve over its whole length. The raphe itself is almost always invisible and its presence can be inferred only indirectly, by the presence of fibulae, which are rib-like, square or rectangular structures. The fibulae are usually small and discrete, but in a few species, they are extended across part or all of the valve. There may or may not be central raphe endings and this is an important taxonomic character; where present, they are usually detectable in LM as a slight thickening of the wall of the subraphe canal at the centre. The raphe systems of the two valves may lie on the same side ('hantzschioid symmetry') or on opposite sides ('nitzschioid symmetry'). Generally two chloroplasts per cell, one towards each pole (very occasionally >2 chloroplasts, in some marine species). Each chloroplast is usually simple and is appressed to one or other valve or to the girdle. The central gap between the chloroplasts is where the nucleus lies.


Peronia Brebisson & Arnott ex Kitton (1868)

Cells cuneate in both valve view and girdle view (heteropolar). Broader pole rounded to subcapitate, narrow pole rounded. Valves differ in raphe structure. One valve with developed raphe extending from each pole to approximately one quarter valve length, other valve with only rudimentary raphe at poles - not always visible. Both valves with very narrow central sternum. Short marginal spines can sometimes been seen.


Pinnularia Ehrenberg (1843)
Ber. Bekanntm. Verh. Königl. Preuss. Akad. Wiss. Berlin, 1843: 45
Cells solitary. Live cells usually with two plate-like chloroplasts lying along the sides of the girdle without obvious pyrenoids. The chloroplast margins may be entire or variously lobed and then extending under the valve face. Some species have a single H-shaped chloroplast with the narrow isthmus under the epivalve and then with one or two pyrenoids. Valves usually linear with bluntly rounded, sometimes slightly subrostrate to subcapitate apices. Raphe central, complex, often in a broad hyaline area; polar fissures hooked, central fissures expanded. Striae alveolate, often coarse, sometimes interrupted at the centre to form a broad hyaline fascia. Girdle bands few, the valvocopula with a row of elongate pores.


Placoneis Mereschkowsky (1903)
Beih. Bot. Centralbl. 15: 13
Cells solitary. Live cells with a single, distinctively shaped chloroplast, having a central longitudinal axis from which lobes diverge and lie under the valve face. A pyrenoid lies in the central bridge. Valves linear to linear lanceolate with bluntly rounded, sometimes protracted apices, subrostrate to subcapitate. Striae uniseriate or biseriate, comprised of small rounded pores. Raphe slit central, slightly expanded at the centre, deflected to the same or opposite sides over the poles. Girdle narrow, comprised of bands with a single row of pores.


Planothidium Round et L.Bukhtiyarova (1996)

Cells are solitary, elliptic, elliptic-lanceolate or lanceolate in outline, with a shallow mantle. In girdle view they are slightly curved often with a prominent thickening around the raphe endings on the lower (concave) valve. The raphe (concave) valve has a narrow linear axial area and a rectangular or "butterfly"-shaped central area. Striae are radial throughout. The rapheless (convex) valve has a similar density of striae to that on the raphe valve, but these are less strongly radiate and, in some cases, almost parallel at the centre of the valve. There is often has a prominent horseshoe-shaped thickening on one side of the rapheless valve.


Psammothidium L.Bukhtiyarova et Round (1996)

Cells are solitary, elliptical, elliptical-lanceolate or linear-elliptical in outline with rounded apices (very rarely rostrate or capitate). They are flexed in girdle view, with a conves rapheless valve and a concave raphe valve. They attach to substrata via the raphe valve face. The raphe valve has fine radiate striae throughout with a narrow, linear or lanceolate axial area and a distinct rectangular or butterfly-shaped central area. The rapheless valve is variable. In some instances it is similar to the raphe valve, save for the absence of the raphe itself. However, there are also taxa whose rapheless valves have broader axial areas and no central area, or which have a narrow axial area and a distinct central area that is often a different shape to that on the raphe valve. In most cases, the central area on the rapheless valve is symmetrical but there are a few cases where it is asymmetrical.


Reimeria Kociolek & Stoermer (1987)

Valves symmetrical to transapical axis, asymmetrical to the apical axis (slightly dorsiventral), with bluntly rounded to slightly subcapitate ends. Dorsal margin convex, ventral margin straight to undulate with central inflation. Raphe more-or-less straight and slightly ventrally located. Single stigma located almost centrally between the proximal raphe ends. Striae coarse and absent from the ventral central area.


Rhoicosphenia Grunow (1860)

Asymmetrical to the transapical axis and symmetrical to apical axis in valve view (heteropolar). Cuneate and flexed in girdle view. Wider pole broadly rounded, narrow pole rounded. Septa often clearly visible in both valve and girdle views. Cells heterovalvate with the concave valve having a fully extended raphe and the convex valve with only very short raphe slits. Raphe more-or-less straight on both valves, with inflated proximal ends.


Rhopalodia O. Müller (1895)
O. Müller (1895). Rhopalodia. Ein neues Genus der Bacillariaceen. Engler's Bot. Jahrb. 22: 54-71. p. 57. nom. cons.
Cells solitary. Frustules isopolar. Cells when alive are most often seen in girdle view. This is effectively also the aspect presented by isolated valves, but the form of the valve (rather like a single orange segment) makes the distinction between valve and girdle view somewhat academic. Valves dorsiventral, with a straight or concave ventral margin and an arched dorsal margin; shape hemi- lanceolate, linear-lanceolate or rhombic. Valve sometimes nicked at the centre, marking the position of the central raphe endings. Valve poles usually cuneate or pointed, but sometimes rounded. Striae almost always easily visible in the light microscope, but individual stria pores sometimes difficult to detect (contrast Epithemia). The valve is traversed by prominent partitions, which appear in valve view as dark bars ('costae'). The partitions span the raphe and act there as fibulae. Raphe system fibulate, appearing to lie at or near the dorsal margin. The raphe itself is difficult to see in the light microscope, but its presence can be detected via the subraphe canal beneath the raphe, which reveals itself as a longitudinal line, running parallel and close to the dorsal margin, on which the partitions appear to rest. One large chloroplast per cell, appressed to the ventral side of the cell and lobed at its edges where it extends into the spaces between the valve partitions. Careful observation of the central part of the cell, especially in girdle view, will reveal the presence of one to several ellipsoidal bodies with ± homogeneous, slightly darker contents, in addition to the nucleus (which is more difficult to detect). These used to be termed 'endosymbiotic cyanobacteria' and are found in all species of Epithemia and Rhopalodia. Recent research, however, suggests that they are a vertically transmitted, permanent endoysmbiotic stage in the transition from a 'true' free-living cyanobacterium to a nitrogen-fixing eukaryotic organelle (Prechtl et al., 2004).


Rossithidium Round et L. Bukhtiyarova (1996)

Linear (occasionally narrowly-elliptical) valves with rounded apices with fine, parallel or slightly radiate striae. The rapheless valve has a narrow, linear axial area whilst a small central area may be present on the raphe valve.


Sellaphora Mereschkowsky (1902)
C. Mereschkowsky (1902). On Sellaphora, a new genus of diatoms. Ann. Mag. Nat. Hist., ser. 7, 9: 185-195. p. 186
Cells solitary, very occasionally forming chains of a few cells, but without linking structures. Frustules isopolar. Mantles and girdle shallow or fairly deep, so that cells can be seen often in valve view or girdle view. Isolated valves usually lie in valve view. Valves bipolar, bilaterally symmetrical, usually quite simple in shape: elliptical, lanceolate to linear, with rounded, broadly subcapitate or capitate poles. Striae almost always visible, mostly slightly radial at the centre, variable elsewhere. Stria pores sometimes visible. Axial area narrow or wide, sometimes also bearing a flap (conopeum) of non-porous silica externally (its margin can be detected as a longitudinal line), sometimes with an external groove. Central area usually present, round to rectangular, sometimes extended across to form a clear band across the whole of the valve (fascia). Special rib-like thickenings (polar bars) are present at each pole in a few of the larger species. Raphe system present, central. Terminal fissures bent towards one side of the valve. Central raphe endings (as seen in LM) usually slightly expanded and deflected a little towards the opposite side from the terminal fissures. Girdle bands non-porous One chloroplast per cell, consisting of two large plates, each appressed to one side of the girdle, connected by a narrow isthmus against one valve. Two spherical granules often present, one in each polar half of the cell, immediately beneath the raphe on one side. A tetrahedral or cushion-like pyrenoid is usually detectable in one side of the chloroplast, close to and within the isthmus linking its two sides.


Semiorbis R.M. Patrick in R.M. Patrick et Reimer (1966)

See S. hemicyclus


Simonsenia Lange-Bertalot (1979)
H. Lange-Bertalot (1979). Simonsenia, a new genus with morphology intermediate between Nitzschia and Surirella. Bacillaria 2: 127-136. p. 131
Cells solitary, small. Frustules isopolar, bilaterally symmetrical. Cells lie in valve or girdle view and isolated valves always in valve view. Valves bilaterally symmetrical, narrowly lanceolate or linear-lanceolate, with pointed or shortly rostrate poles. Striae clearly visible in LM but stria pores unresolvable (in fact the striae are biseriate, with alternating tiny pores visible only in EM). Raphe system fibulate (the raphe itself is impossible to detect in LM), marginal. The fibulae are unlike those found in Nitzschia, because they are not solid bridges of silica but hollow structures, containing an extracellular tube that passes beneath the raphe, creating a narrow passage through from valve face to mantle. Stria pores penetrate the tube. This structure, which can only be seen with EM, is reminiscent of the structure of some Surirella species, though the similarity is probably a homoplasy. In LM, the narrow spaces between the fibulae, giving access from the subraphe canal to the cell lumen, are more obvious than the fibulae themselves, appearing as small dark 'arrowheads'. Central pair of fibulae no more widely separated than the others; central raphe endings absent. In a frustule, the raphe systems of the two valves lie on opposite sides ('nitzschioid symmetry') Chloroplast morphology unknown: the single species is small and easily overlooked when alive.


Stauroneis Ehrenberg (1843)

Cells symmetrical to the apical and transapical axes. Valves elliptical (rarely) to lanceolate, some species undulate in outline. Apices rounded to capitate. Raphe straight. Striae often fine but visible and punctate. Central area with "bow tie" shaped stauros comprising of the non-striated, thickened central nodule and usually extending to the valve margin. Pseudosepta may be present in some species and can be seen in valve view at the apices. Two chloroplasts lying along the girdles and extending under the valve faces.


Stenopterobia Brébisson ex Van Heurck (1896)
H. Van Heurck (1896). A treatise on the Diatomaceae. W. Wesley, London. p. 374
Solitary. Frustules isopolar. Cells seen in valve view or girdle view, straight or sigmoid. Isolated valves almost always lying in valve view. Valves elongate, isopolar, straight or sigmoid; linear or linear-lanceolate. Runnng around the whole perimeter of the valve is a raphe system that opens inwardly into a tubular canal ('canal-raphe'), which is raised on a ridge or keel. The canal is separated from the remainder of the cell interior by fused infoldings of the valve face and mantle, which sometimes create small extracellular 'windows' (fenestrae) beneath the raphe. The subraphe canal communicates with the cell lumen via narrow tubes. In the light microscope, the raphe itself is almost impossible to detect: its presence must be deduced from the presence of the subraphe tubes and the fibulae/fenestrae between them. Striae visible, transverse, interrupted along the midline by a narrow hyaline space or line. Stria pores are very small and undetectable in LM. Raphe system fibulate, containing two raphe slits, each running around half of the valve perimeter. There are therefore two sets of raphe endings, one at each end of the valve. Within a frustule, the raphe endings of the two valves are not offset as in Campylodiscus. One set of raphe endings is apparently equivalent to the central raphe endings of other raphid diatoms; the other set is apparently equivalent to the polar raphe endings, brought next to each other by reflexing of the valve poles in dorsiventral forms perhaps resembling extant Auricula. They are apparently indistinguishable in LM and EM. There is one chloroplast per cell, consisting of two elongate plates, connected by a narrow isthmus near one end of the cell. The plates lie against the valves and, when seen in valve view, appear to fill all except the cell periphery. The median hyaline area is only superficially similar to the sternum or raphe-sternum of other pennate diatoms. Valve development in Surirellales, as in other raphid diatoms except Eunotiales, occurs from the raphe system outwards.


Surirella Turpin (1828)
P.J.F. Turpin (1928). Observations sur le nouveau genre Surirella. Mém. Mus. d'Hist. Nat. Paris 16: 361-368. pp. 362-3
Solitary. Frustules isopolar or heteropolar. Cells seen in valve view or girdle view, depending on the species; isolated valves usually lying in valve view. Occasionally cells are spiralled. Valves bipolar in structure, with a definite longitudinal axis, but may be elliptical or even circular in shape; more often, however, valves are linear, lanceolate or rhombic. Heteropolar or isopolar. Running around the whole perimeter of the valve is a raphe system that opens inwardly into a tubular canal ('canal-raphe') raised on a ridge or keel. The canal is separated from the remainder of the cell interior by siliceous bridges (fibulae) and often also by infoldings of the valve face and mantle, which, in some species where the keel is especially high, fuse to create extracellular windows (fenestrae) beneath the raphe. In the light microscope, the raphe itself is almost impossible to detect: its presence must be deduced from the presence of the fibulae or fenestrae. The valve pores are usually very small and the striae that they form are also sometimes inconspicuous. Much more obvious are rib-like thickenings and wavy infoldings of the valve, the areas of pores and striae that these surround, and the fibulae. Raphe system fibulate, containing two raphe slits, each running around half of the valve perimeter. There are therefore two sets of raphe endings, one at each end of the valve. Within a frustule, the raphe endings of the two valves are not offset as in Campylodiscus. One set of raphe endings is apparently equivalent to the central raphe endings of other raphid diatoms; the other set is apparently equivalent to the polar raphe endings, brought next to each other by reflexing of the valve poles in dorsiventral forms perhaps resembling extant Auricula. There is one chloroplast per cell, consisting of two large plates, often lobed at their margins, connected by a narrow isthmus that lies at one end of the cell, or close to the nucleus in a subcentral position. The plates lie against the valves and, when seen in valve view, appear to fill all except the cell periphery. Along the longitudinal axis of the valve there is sometimes a median rib or hyaline area. This is only superficially similar to the sternum or raphe-sternum of other pennate diatoms. Valve development in Surirellales, as in other raphid diatoms except Eunotiales, occurs from the raphe system outwards.


Tryblionella W. Smith (1853)
W. Smith (1853). A synopsis of the British Diatomaceae, vol. 1. J. van Voorst, London. p. 35
Cells solitary. Frustules isopolar. Cells and valves seen in valve or girdle view, depending on species, but more often in valve view. Valves structurally asymmetrical, because the raphe system is always marginal. Valve outline ± symmetrical, linear to lanceolate or elliptical, often quite wide relative to the related genus Nitzschia, sometimes slightly constricted. There is sometimes a central 'nick' on one side, marking the position of the central raphe endings. Poles simply rounded, cuneate, to subrostrate; rarely subcapitate. Transverse striae usually resolvable, but often difficult to see (except the acuminata-apiculata-hungarica and navicularis-punctata groups). Valve face either with pronounced longitudinal folding and often also one or more longitudinal sterna (longitudinal areas of valve without pores and usually thickened), or with a single well-differentiated axial sternum. Margin distal to the raphe often bearing a small or large marginal ridge, which can often be detected in LM by careful focusing. Sternum or other parts of the valve face sometimes bearing pronounced, rib-like thickenings, which are much obvious than the striae and interstriae (virgae) and may indeed be mistaken for them. Raphe system fibulate, marginal. The raphe itself is invisible and its presence can be inferred only indirectly, by the presence of fibulae, which are never extended across the valve but are short bars or large square or rectangular structures; in some species the fibulae appear striated in LM, apparently reflecting the fusion of several smaller elements. Central raphe endings are usually present (but not in the navicularis-punctata group); they are usually detectable in LM as a slight thickening of the wall of the subraphe canal at the centre. The raphe systems of the two valves lie on opposite sides ('nitzschioid symmetry') Two chloroplasts per cell, one towards each pole. Each chloroplast is usually simple and is appressed to one or other valve. The central gap between the chloroplasts is where the nucleus lies.

Comments to: Diatom Key Development Team