GENERA starting with "H"



  1. Spines two per valve, elongated
  2. Frustules lightly silicified
  3. Numerous scale-like girdle bands

Centric - Acanthoceras is a solitary, planktonic genus. Cells are usually seen in girdle view, because of the numerous girdle bands and spines creating a large length to breadth ratio. This centric diatom appears somewhat rectangular in shape, with two spines, or setae, extending from each valve. Living cells contain four small, plate-like chloroplasts. Populations of Acanthoceras form ephemeral summer blooms in shallow,...

Achnanthes 9

  1. Frustules heterovalvar - one valve with a raphe, the other lacking a raphe
  2. Raphe valve usually possesses a fascia or stauros
  3. Valves shallow and arched in girdle view
  4. Rapheless valve without fascia, narrower sternum usually off center

Monoraphid - The frustules of Achnanthes are heterovalvar. The raphe valve usually possesses a central area of thickened silica, called a fascia or stauros. The rapheless valve has no such central area and the sternum may be positioned near the valve margin. In girdle view, the mantle of the rapheless valve appears to be more ornamented than that of the raphe valve. The striae are uni-, bi-, or triseriate and composed of areolae...

Achnanthidium 17

  1. Frustules heterovalvar - one valve with a raphe, the other valve lacking a raphe
  2. Raphe valve concave, rapheless valve convex
  3. Valves very small, linear-elliptic to lanceolate
  4. Striae usually uniseriate
  5. Raphe with terminal fissures

Monoraphid - Frustules are heterovalvar, that is, one valve possesses a raphe, while the other valve lacks a raphe. Species of Achnanthidium are generally small in size, with narrow valves (less than 30 µm in length and 5 µm in breadth). The shape of the valves differs by species, but the ends may be rounded, capitate or rostrate. In girdle view, the frustules appear arched. The raphe valve face is concave, while that of the...

Actinella 1

  1. Valves distinctly asymmetric
  2. Raphe highly reduced
  3. Helictoglossae distinct
  4. Spines present

Eunotioid - Cells of Actinella grow singly, or in small colonies that are joined by mucilage secreted at the narrow ends of the cells. Valves are distinctly asymmetric to the apical and transapical axes. The raphe is highly reduced and is restricted to the ventral margins of valves. Helictoglossae are distinct as dark, silicious thickenings on the ventral margins. Small to large spines are present on the valve margins. The species...

Actinocyclus 1

  1. Pseudonodulus present (may be visible only with SEM)
  2. Several rimoportulae present at margin of valve face

Centric - Frustules of Actinocyclus are centric in outline with a relatively narrow mantle. The valve face may be flat or may have concentric undulations in the valve surfaces. Internal openings of rimoportulae are often visible in LM. The genus is distinguished by the presence of a pseudonodulus, a structure found in the three genera Actinocyclus, Hemidiscus and Roperia. The pseudonodulus may only be visible through use of SEM....


  1. Terminal raphe fissures strongly bent
  2. Externally, areolae covered by hymenes
  3. Striae radial, uniseriate

Symmetrical biraphid - Adlafia is a genus characterized by small cells, usually less than 25 µm in length. The valve shape is linear to linear-lanceolate. Adlafia is distinguished by terminal raphe fissures that are strongly bent and angular. The shape of the valve ends may be blunt, rostrate, or subcapitate. Striae are radial. Externally, the areolae are covered by hymenes. The striae are uniseriate. Species in this genus are also...


  1. Valves dorsiventral, curved along pervalvar axis
  2. Apical pore field present at the footpole
  3. Septum and pseudoseptum present, small

Asymmetrical biraphid - SPECIES OF AFROCYMBELLA ARE NOT KNOWN TO OCCUR IN NORTH AMERICA The valves of Afrocymbella are asymmetrical to both the apical and transpical axis. In addition, the valves are curved along the pervalvar axis. An apical porefield is present at the footpole, which is bisected by the terminal raphe. The raphe is dorsally deflected at the proximal end and ventrally deflected at the terminal end. One, or rarely two, dorsal...


  1. Margins distinctly undulate
  2. Raphe short

Eunotioid - Amphicampa valves are asymmetric to the apical axis and symmetric to the transapical axis. Both the dorsal and ventral margins of the valves are undulate. The raphe is very short and restricted to the valve ends. The raphe may only be visible through the thickening of the helictoglossa. A single species, A. eruca Ehrenberg, is found in circumneutral to alkaline waters in the midwest and western United States. This...

Amphipleura 1

  1. Median rib present
  2. Medium rib split at the poles, forming 'needle eyes'
  3. Raphe short compared to other naviculoid genera

Symmetrical biraphid - Amphipleura is distinguished by the narrow, median rib that extends along the long axis of the valve. This rib is formed on the internal face of the valve, a feature observable in SEM. Near the valve apices, the rib forms two branches, or ‘needle eyes’, around the raphe system. The raphe of Amphipleura is short, limited to the area within the “needle eyes”. The valves of Amphipleura are linear or spindle-shaped. The...

Amphora 4

  1. Raphe eccentric, positioned along ventral margin
  2. Dorsal margin of valve deeper than ventral margin
  3. Raphe ledge on both sides of raphe
  4. Striae on dorsal margin interrupted
  5. Dorsal fascia present

Asymmetrical biraphid - Valves are asymmetrical to the apical axis and symmetrical to transapical axis. On the dorsal margin, the valve mantle is deeper than on ventral margin. As a result, the frustule is wedge-shaped, similar to a section of an orange. This wedge shape prevents complete focus with a microscope in one focal plane. In most species of Amphora sensu stricto, there is an abrupt differentiation between the valve face and mantle by...


  1. Valves slightly, to stongly, asymmetric to the apical axis
  2. Valve symmetric to transapical axis
  3. Rimoportulae two per valve
  4. Valves greater than 60 µm in length

Eunotioid - Valves of Amphorotia are large in size, generally greater than 60 µm in length. The valves are asymmetric to the apical axis and symmetric to the transapical axis. In addition, the frustules are asymmetric to the transapical plane, so that the dorsal mantle is very broad as compared to the ventral mantle. Each valve has two rimoportulae, positioned near the apices. The areaolae are coarse and not well organized into...

Aneumastus 4

  1. Areolae complex, opening into deep pits internally (SEM)
  2. Valves lanceolate, often with capitate or rostrate ends
  3. Valve face flat
  4. Valve mantle narrow

Symmetrical biraphid - Species of the genus Aneumastus are typically lanceolate, with capitate or rostrate ends. The valve face is flat, with a narrow valve mantle. The striae are uniseriate over most of valve face, biseriate near the valve mantle and composed of complex areolae. The areolae open into deep pits internally, a feature visible in SEM. The raphe sternum is narrow, but widens centrally to form an elliptical or rectangular central...

Anomoeoneis 1

  1. Striae distinctly punctate and irregular
  2. Central area extends to valve margins
  3. Distal raphe ends deflected and usually distinct

Symmetrical biraphid - Anomoeoneis valves are lanceolate to elliptical-lanceolate in outline, with broadly rounded to capitate apices. The central area may be symmetrical or asymmetrical, extending unilaterally to valve margins. The distal raphe ends are deflected and usually distinct. The irregular striae and hyaline areas on both sides of the axial area distinguish Anomoeneis from Brachysira and Navicula. The chloroplast is a single...


  1. Frustule heterovalvar - raphe and rapheless valves
  2. Sternum eccentric
  3. Striae composed of radiate, curved areolae

Monoraphid - Anorthoneis is primarily a marine genus of littoral, epiphytic or epipsammic habitats in coastal areas of temperate and warm water. One species, A. dulcis Hein, was described from freshwater rivers in northern Florida. The species has also been reported from rivers in Baja California, Mexico. The morphological features are documented here for this freshwater species, rather than for the genus as a whole. Valves of A....

Asterionella 1

  1. Valve linear-lanceolate with capitate ends
  2. Spines present on valve margins
  3. Cells linked by mucilage pads forming characteristic stellate colonies
  4. Porefield one, or two
  5. Rimoportula(e) at headpole and/or footpole

Araphid - The valves of Asterionella are symmetrical to the apical axis and asymmetric to the transapical axis. In valve view, the valves are linear-lanceolate in shape, with capitate ends. Living cells attach by mucilage pads at the basal ends, or footpoles, to form characteristic stellate colonies. Living colonies present cells in girdle view, while processed samples break up the colonies so that single valves typically lie in...

Aulacoseira 19

  1. Valve mantle deep, cells usually seen in girdle view
  2. Ringleiste may be present
  3. Frustules form colonies joined by spines
  4. Collum may be present

Centric - The centric frustules of Aulacoseira are linked to one another by spines to form filaments. Cells are typically seen in girdle view, because of the deep valve mantle. Cells often form colonies and, depending on the species, may be joined by linking spines. The shape of the linking and separation spines and relationship between spines and striae are important characters that distinguish species within Aulacoseira....

Bacillaria 1

  1. Raphe positioned in the central apical axis
  2. Striae relatively coarse
  3. Cells grow in colonies, linked by their keels

Nitzschioid - The raphe of Bacillaria species is located within a raised keel. In contrast to the excentrically positioned raphe and keel of Nitzschia, the raphe of Bacillaria is centrally positioned along the apical axis. The striae are distinct and relatively coarse. Living cells form colonies. The colonies are joined by silica hooks near the keel, and these silica hooks allow the cells to slide past one another in coordinated...

Biremis 1

  1. Valves with linear margins
  2. Striae not continuous across valve face
  3. Striae chambered, open externally in circular or slit-like foramina
  4. Valve mantle with similar chambered striae

Symmetrical biraphid - Biremis currently contains both freshwater and marine species. Freshwater species possess a linear margin and bilateral symmetry, while marine species are strongly dorsiventral, with amphoroid symmetry. The striae of Biremis appear to be “interrupted” (they are not continuous), due to internal plates that cover marginal chambered striae. Internally, the striae are chambered, while externally they open in circular or...

Boreozonacola 2

  1. Valves linear, often with triundulate margins
  2. Areolae coarse
  3. Central area oval to round

Symmetrical biraphid - Valves of Boreozonacola are linear, often with tumid central and subapical margins. The raphe is lateral. The proximal raphe ends are simple, curving to the primary side of the valve. The distal raphe ends curve to the secondary side of the valve. The axial area is narrow and linear. The central area is round. Striae are radiate and are composed of coarse areolae. Voight discontinuities are often present. The genus...

Brachysira 10

  1. Valves linear to linear-lanceolate to cruciform
  2. Valve apices rounded to protracted
  3. Striae finely punctate, form longitudinal undulations

Symmetrical biraphid - Valves of Brachysira are linear to linear-lanceolate in outline. The ends of the valves, or apices, may be rounded to protracted. In general, the valves are symmetrical to the apical axis. Some specimens may have a slight, to strong, asymmetry to the transapical axis. The striae are finely punctate, and form longitudinal undulations. The raphe is straight and the axial area is narrow. In the SEM, an elevated siliceous...

Brebissonia 1

  1. Striae biseriate
  2. Proximal raphe ends relatively widely separated

Asymmetrical biraphid - Valves are rhombic-lanceolate or lanceolate and slightly isopolar. The valve face is flat and the mantle is shallow. Striae are biseriate. Robust transapical ribs are present. The proximal raphe ends are expanded and internally deflected to the primary side of the valve. The distal raphe fissure is long and bent. Internally, a prominent helictoglossa is present. Copulae are open. Brebissonia is considered to be closely...


  1. Frustules extremely small
  2. Raphe reduced

Symmetrical biraphid - Frustules are extremely small, less than 15 µm in length. Valves are broadly lanceolate, with small, capitate apices. The raphe is linear and short. The raphe is contained within a distinctly thickened sternum. Striae are slightly radiate in the center valve, becoming parallel near apices. Striae are composed of single row of 1-2 areolae, a feature visible in SEM. Externally, the proximal and distal raphe fissures are...


  1. Valve face with central areolate area surrounded by radiating striae
  2. Rimoportulae or fultoportulae absent
  3. Valve margins with small, tapered spines

Centric - Brevisira is a centric diatom, with central areolae surrounded by radiating striae on the valve face. In contrast to many centric genera, Brevisira lacks both rimoportulae and fultoportulae. The frustules of Brevisira are small, with small tapered spines on the valve margins. Within a single frustule, one valve face may be concave, while the other is convex. The valve mantle is also ornamented with areolae. Living cells...

Caloneis 4

  1. Striae chambered
  2. One or two longitudinal lines may be present
  3. Central area may have lunate or irregular thickened areas

Symmetrical biraphid - Species of Caloneis are characterized by chambered striae, like those of Pinnularia. The chambered striae give the appearance of one to two longitudinal lines. The striae of Caloneis are composed of fine alveoli, also like Pinnularia. The central area of Caloneis may be expanded, and may include silica thickenings that are lunate or irregular in shape. Living cells possess one or two plastids. Several species within...

Campylodiscus 1

  1. Valve 'saddle' shaped
  2. Raphe runs along entire valve margin
  3. Raphe positioned in marginal keel

Surirelloid - Frustules of Campylodiscus possess a complicated three-dimensional structure which is often termed a distinctive “saddle shape”. Like the other genera within the family Surirellaceae, the raphe system is raised onto a keel and positioned along the entire valve margin. Cells possess a single chloroplast, with two lobed margins. Cells grow singly in epipelic habitats of freshwaters, brackish waters and marine...

Capartogramma 1

  1. Central region X-shaped
  2. Pseudosepta present
  3. Axial area narrow
  4. Striae uniseriate, punctate

Symmetrical biraphid - Capartogramma is characterized by a very distinctive “x-shaped” thickening of silica near the central part of the valve. Frustules are generally lanceolate in valve outline. Pseudosepta are present at the valve apices. The raphe is positioned within a narrow axial area. Striae are uniseriate and punctate. Capartogramma is monotypic in North America (C. crucicula [Grunow ex Cleve] Ross). This species is found in rivers...

Cavinula 8

  1. Valves linear-lanceolate to nearly circular
  2. Striae uniseriate
  3. Striae radial
  4. Terminal raphe fissures absent

Symmetrical biraphid - Species of Cavinula are characterized by uniseriate, radial striae composed of round to elongate areolae. Valves are linear-lanceolate to nearly circular in outline. Internally, the central sternum is thickened, a feature visible in SEM. Externally, the proximal raphe is expanded, while the internal proximal raphe ends are straight. The terminal raphe fissures are absent. One or two chloroplasts are present in living...


  1. Frustules with long, thin spines
  2. Spines overlap to form colonies of several cells

Centric - Chaetoceros is a centric diatom with very lightly silicified frustules. Each frustule possesses four long, thin spines, or setae. The setae link the frustules together to form colonies of several cells. Frustules are usually seen in girdle view. Chaetoceros is primarily a marine genus, with only a few representatives in inland waters of the United States. Inland cells reach their greatest abundances in saline or brine...

Chamaepinnularia 3

  1. Striae composed of chamber-like areolae
  2. Externally, areolae covered by vela

Symmetrical biraphid - The valve margins of Chamaepinnularia are linear, nearly linear, or undulate. Cells are small, usually less than 25 µm in length and 4 µm in breadth. The striae are composed of simple, chamber-like areolae, with external openings covered by vela. Internally, the areola openings are divided with internal plates of silica. The raphe system may be simple or complex in structure, as in Pinnularia or Navicula. Externally,...

Cocconeis 6

  1. Frustules heterovalvar - one valve with a raphe, the other lacking a raphe
  2. Valves elliptic
  3. Striae uniseriate in freshwater species
  4. Terminal raphe fissures absent

Monoraphid - Cocconeis is heterovalvar, that is, the raphe valve ornamentation differs from that of the rapheless valve. Ornamentation of the raphe valve may differ markedly from that of the rapheless valve. The valve mantle is narrow in relation to the valve face, so cells are rarely (if ever) seen in girdle view. Valves may be flexed, or arched, along the apical axis forming a ‘saddle’ shape. Striae are often uniseriate, but some...

Cosmioneis 3

  1. Striae uniseriate and radial
  2. Valve face flat, with broad valve mantle
  3. Central area elliptical
  4. Internal proximal raphe ends anchor-shaped
  5. External proximal raphe ends expanded

Symmetrical biraphid - Valves of species in the genus Cosmioneis are lanceolate to elliptical in outline, with capitate to rostrate ends. The valve face is flat, and the valve mantles are relatively broad. Striae are uniseriate and radial, and are composed of round to oval areolae. Striae become irregular near the central area. The axial area is narrow and linear, except at the central area, where the central area is expanded and ellipitical....


  1. Stauros narrow
  2. Frustules constricted in girdle view

Symmetrical biraphid - SPECIES OF CRASPEDOSTAUROS ARE NOT KNOWN TO OCCUR IN NORTH AMERICA Valves of Craspedostauros are linear, with a narrow central stauros. In girdle view, frustules are constricted around the central stauros. Cells often lie in girdle view, because of the deep girdle. Numerous copulae, or girdle bands, are present. Areolae are cribrate. Live cells have two lobed chloroplasts. Species within Craspedostauros were previously...

Craticula 15

  1. Striae parallel, or nearly parallel
  2. Valves lanceolate
  3. Central area small or absent
  4. Distinct internal valves may be present

Symmetrical biraphid - The valves of Craticula are lanceolate. The striae are distinctly parallel, or nearly parallel. The central area is greatly reduced or absenct. The genus is polymorphic, forming different valve morphologies. There are three valve types found for this taxon: vegetative, “heribaudii”, and craticula forms. The three forms are produced as a cellular strategy for surviving conditions such as desiccation (Schmid 1979)....

Ctenophora 1

  1. Valve elongate, linear-lanceolate
  2. Central area a thickened fascia with ghost striae
  3. Rimoportulae present at each pole
  4. Rimoportula positioned diagonally

Araphid - Ctenophora frustules are elongate and linear-lanceolate in valve outline. The central part of the valve has a thickened area, or fascia, with faint, ghost striae. Each pole contains a rimoportula, positioned near the axial area. The rimoportula is positioned diagonally. Living cells grow in tuft-like colonies, attached to surfaces by mucilage pads. The genus is common in brackish waters, including sites that have...

Cyclostephanos 2

  1. Striae fasciculate, extending from valve face to mantle
  2. Costae form 2 branches on mantle
  3. Rimoportulae positioned near mantle

Centric - The striae of Cyclostephanos are organized into bundles, or fascicles, that extend from the valve face onto the mantle. The costae, or thickened ribs, also extend from the valve face to mantle. The costae branch on the valve mantle below the plane of the marginal spines, a key feature of the genus. Marginal rimoportulae are positioned near the spines, if spines are present. The branching costae distinguish this genus...

Cyclotella 2

  1. Valve with differential ornamentation between the central and marginal area
  2. Central area flat to tangentially undulate
  3. Single rimoportula positioned on a costa

Centric - In the genus Cyclotella sensu stricto the rimoportula is positioned on a costa, within the ring of marginal fultoportulae (Alverson et al. 2011). Like the genera Cyclostephanos, Discostella and Lindavia the valve face of Cyclotella has differential ornamentation between the central and marginal areas. The central area of the valve face may be flat to tangentially undulate and smooth to colliculate. Numerous marginal...


  1. Frustules very narrow, elongate, with drawn-out ends
  2. Frustules twisted
  3. Frustules very lightly silicified

Nitzschioid - Cylindrotheca cells are solitary and are characteristically needle-like, thin and elongated in shape. The ends of the cell tend to be drawn out from the center part. The frustules are only lightly silicified. The valves, including the keel and raphe canal, wrap around each other, forming a twisted frustule. As a result of the strong twist of the raphe system, the cells rotate and twirl as they move. The girdle bands are...

Cymatopleura 2

  1. Frustules elliptical to panduriform
  2. Valve face undulate
  3. Raphe positioned in marginal keel

Surirelloid - Cymatopleura cells are generally large in size, often over 200 µm in length. The valves are elliptical to panduriform. Species in Cymatopleura possess characteristic undulations on the valve face. In girdle view, Cymatopleura is distinct because of the undulating valve face. The raphe is located within a shallow keel, positioned along entire valve margin. In living cells a single plastid, composed of two plates is...

Cymbella 13

  1. Valves slightly to strongly asymmetric to the apical axis
  2. Valves symmetric to the transapical axis
  3. Distal raphe ends deflected dorsally
  4. Stigmata, if present, located on the ventral side

Asymmetrical biraphid - The valves of Cymbella are slightly to strongly dorsiventral, or asymmetric to the apical axis. Valves are symmetric to the transapical axis. The terminal raphe fissures are deflected to the dorsal side, an important diagnostic feature. Apical pore fields are present at both poles. Striae are uniseriate. One or more stigmata may be present. If a stigma or stigmata are present, they are located on the ventral side of the...

Cymbellonitzschia 1

  1. Valves slightly to strongly asymmetric to the apical axis
  2. Raphe positioned on ventral margin
  3. Cells small

Nitzschioid - Cymbellonitzshia possesses a raphe on the ventral margin of the valve, similar to the genus Nitzschia. In contrast to Nitzschia, however, the valves of Cymbellonitzschia are slightly to strongly asymmetric to the apical axis. The striae are uniseriate. The cells of Cymbellonitzshia are small. Cymbellonitzshia is an uncommon genus that grows on sand and sediment surfaces.

Cymbopleura 32

  1. Valves slightly asymmetric to apical axis
  2. Valves symmetric to transapical axis
  3. Terminal raphe fissures deflected dorsally
  4. Valves lack stigmata and apical pore fields

Asymmetrical biraphid - Cymbopleura frustules are characteristically large and relatively broad in valve outline. The frustules of Cymbopleura are asymmetric to the apical axis, typically with a convex ventral margin and an even broader convex dorsal margin. The striae are composed of rounded areolae. The proximal raphe ends are expanded, while the terminal raphe fissures are hooked and dorsally deflected. Species within Cymbopleura lack apical...

Decussata 1

  1. Valves linear to linear-lanceolate
  2. Striae punctate
  3. Striae decussate

Symmetrical biraphid - Decussata valves are linear to linear-lanceolate, with slightly rostrate ends. The striae are punctate, comprised of distinct, round areolae. The striae are decussate, meaning they appear to be organized into rows - a transapical row and two oblique rows. Decussata includes two species, including the former Navicula placenta. Decussata placenta grows in low nutrient, slightly acidic seeps in Iowa. It is also known from...


  1. Valves narrow, slightly asymmetrical to apical axis
  2. Valves symmetrical to the transapical axis
  3. Terminal raphe fissures deflected dorsally
  4. Stigmata and apical pore fields absent

Asymmetrical biraphid - Delicata valves are slightly asymmetrical to the apical axis and symmetrical to the transpical axis. The valve shape is narrow and lanceolate. The raphe is distinctly lateral and becomes reverse-lateral at the proximal ends. The terminus of the raphe is thin, comma shaped, and terminal raphe fissures are deflected toward the dorsal side. Valves lack a distinct central area. Stigmata are absent. The striae are fine and...

Denticula 3

  1. Raphe eccentric, on a wide, low marginal keel
  2. Internal fibulae distinct, thickened

Nitzschioid - The raphe system of each valve is positioned diagonally opposite to one another, demonstrating nitzschiod symmetry. The raphe system is located within a wide, low keel. The raphe may or may not have proximal raphe ends or be continuous across the valve, a feature difficult to see without SEM. Thick internal bars of silica (fibulae) are present and parallel to the striae. The fibulae widen near the valve margins to form a...

Diadesmis 1

  1. Areolae elongate
  2. Frustules small, usually less than 20 µm in length
  3. Cells form band-like colonies

Symmetrical biraphid - Diadesmis is biraphid, although the raphe often becomes secondarily filled with silica. Frustules of Diadesmis are small, usually less than 30 µm in length. The striae are composed of areolae elongated in the transapical direction, a characteristic that may be difficult to discern in the LM. Frustules form band-like colonies, which may be linked by marginal spines. The raphe systems of many frustules are secondarily...

Diatoma 5

  1. Transverse costae distinct
  2. Striae uniseriate
  3. Single rimoportula

Araphid - The genus Diatoma possesses characteristic thickened transverse costae, septae are absent. Each valve has a single rimoportula, positioned near a valve terminus and oriented transapically. Furthermore, a raised central sternum is present. Under SEM, Diatoma has a ligula on the second girdle band that attaches to the polar position of the valve, adjacent to the interlocking valvocopula. The phylogenetic relationships...

Diatomella 1

  1. Valves with distinct septa
  2. Striae short
  3. Proximal raphe ends expanded

Symmetrical biraphid - The frustules of Diatomella are symmetrical to the apical and transapical axes. The valve outline is linear-elliptical. An internal septum is present, with three openings forming a distinctive internal thickening of silica. The striae are short, often not extending far beyond the valve margin. The proximal raphe ends are expanded. Diatomella is the only naviculoid diatom that possesses a septum (rather than a...

Didymosphenia 1

  1. Valves large and robust
  2. Stigmata one to several
  3. Apical porefield at footpole, large

Asymmetrical biraphid - Frustules of Didymosphenia are asymmetrical to the transapical axis and symmetrical to the apical axis (although some populations may be slightly to strongly asymmetrical to the apical axis). One to several stigmata are present, a feature that may be variable within a given species. A large apical porefield is present at the footpole. The terminal raphe fissures are deflected prior to reaching the apical porefield....

Diploneis 3

  1. Valves elliptical to panduriform
  2. Longitudinal canals present
  3. Areolae complex

Symmetrical biraphid - Frustules of Diploneis are typically elliptical to panduriform, with bluntly rounded apices. Each valve possesses two longitudinal canals, one on each side of the raphe. The canals are positioned within the silica cell wall and open to the exterior through pores, but lack openings to the interior of the cell. The function of these canals is uncertain. The frustules are heavily silicified, with complex (loculate)...

Diprora 1

  1. Frustules typically with a convex and concave valve
  2. Frustules joined to form filaments
  3. Marginal pores present
  4. Apical prows present
  5. Cingulum short, band-like

Araphid - The genus Diprora is monotypic (contains a single species) and, to date, is known only from the state of Hawai’i. The genus is tentatively placed in the Fragilariophyceae, although the relation of Diprora to other related taxa is uncertain. This taxon is somewhat similar to the marine taxon, Hyalinella, although Diprora lacks an ocellus-like structure. The filamentous araphid morphology suggests an affinity to the...

Discostella 2

  1. Central area with large alveoli, often star-shaped
  2. Marginal radial costae
  3. Marginal fultoportulae present

Centric - Discostella grows as solitary cells or forms colonies in chains. The central area may be flat or concentrically undulate. Large, distinctive alveoli form a star-shaped pattern in the central valve. Radial costae are present at the valve margin. Small spines may be present or absent on the costae. Central to the costae are striae, which may be uniseriate or multiseriate. Marginal fultoportulae are present, located on...

Distrionella 1

  1. Valves elongated, with capitate ends
  2. Striae unevenly spaced
  3. Rimoportula single, often central within a stria near one pole

Araphid - Frustules of Distrionella are elongated with capitate ends. Valves may be slightly asymmetrical to the transapical axis. Striae are unevenly spaced and become scattered and more irregular at the valve ends. Transapical ribs often evident. A central sternum is absent. Porefields, composed of simple porelli, are present at both valve ends. One rimoportula is present, and is usually positioned centrally within a stria near...

Ellerbeckia 1

  1. Valve surface flat
  2. Unique tube-like processes
  3. Frustules joined in colonies

Centric - Ellerbeckia frustules are large cylinders with relatively narrow mantles. Cells are joined into filamentous colonies, linked by an interlocking series of ridges and grooves on opposing valve faces. The valve surface is flat and lacks the pores or processes of many centric diatom genera. Two different kinds of valves comprise a colony: linking valves that connect adjacent cells within a colony, and separation valves that...

Encyonema 14

  1. Dorsal margin highly arched
  2. Ventral margin straight or nearly straight
  3. Stigmoids present or absent, if present then located on the dorsal central area
  4. Distal raphe ends ventrally deflected

Asymmetrical biraphid - Encyonema frustules are asymmetrical to the apical axis and symmetrical to the transapical axis. The dorsal margin is highly arched, while the ventral margin in straight or nearly straight. Stigmoids may be present or absent. If a stigmoid is present, it is located on the dorsal side of the central area. Distal raphe ends are ventrally deflected, while distal raphe ends of Cymbella are dorsally deflected. Apical pore...

Encyonopsis 18

  1. Valves naviculoid, slight apical asymmetry
  2. Areolae round or transapically elongate
  3. Striae radiate or parallel at the apices
  4. Proximal raphe ends deflected dorsally
  5. Distal raphe fissures deflected ventrally

Asymmetrical biraphid - Valves are naviculoid and slightly dorsiventral, symmetric to the transapical axis and asymmetric to the apical axis. The raphe is centrally located. Areolae in the striae are round or oblong and oriented with their long axes parallel to the transapical axis. Transapical striae are radiate near the valve center and radiate or parallel near the apices. Proximal raphe ends are deflected toward the dorsal margin. Distal...

Entomoneis 4

  1. Bilobate, raised keel
  2. Valves torsioned
  3. Raphe positioned within raised keel

Asymmetrical biraphid - The frustules of Entomoneis are structurally complex, with numerous girdle bands. Valves are lanceolate or linear with acute apices. In processed material, cells usually come to rest in girdle view, although many different viewing aspects are possible. Cells are rarely viewed in valve view (but see image with #2). The valve face is laterally compressed but highly arched to form a thin bilobate wing, or keel. The cells...

Envekadea 3

  1. Raphe straight
  2. Distal raphe ends deflected to opposite sides

Symmetrical biraphid - Species within the genus Envekadea are naviculoid, with a sigmoid raphe. The areolae are large and composed of rectangular to irregular polygons. Externally, the areolae are occluded by hymenes. The copulae are unoramented. A single chloropolast is present, which is H-shaped in valve view. Envekadea includes the former Navicula hedinii Hustedt and N. pseudocrassirostris Hustedt. Envekadea species been documented from...


  1. Cells small
  2. Sternum present, central

Symmetrical biraphid - The cells of Eolimna are small, generally less than 20 µm in length. A narrow central sternum is present, and the raphe is straight. Valves lack apical pore fields. SEM features show areolae are positioned medially within the frustule wall and are covered internally by hymens. Eolimna includes the former Navicula minima and N. adnata.

Epithemia 5

  1. Raphe eccentric, positioned along ventral margin
  2. Raphe opens internally into a canal
  3. Areolae complex

Epithemioid - Epithemia has an eccentric raphe system, positioned along the ventral margin. Each branch of the raphe is arched toward the dorsal margin. Externally, the proximal raphe slits terminate in expanded ends, while internally, the raphe slit is continuous through the central nodule. The raphe is also supported internally by large, transapical costae. The valvocopulum, which is the girdle band next to the valve mantle, often...

Eucocconeis 4

  1. Frustules heterovalvar - one valve with a raphe, the other lacking a raphe
  2. Frustules with torsional twist about the apical axis
  3. Valves linear-elliptic to lanceolate
  4. Striae uniseriate
  5. Terminal raphe fissures long, curved to opposite sides

Monoraphid - Eucconeis frustules are bent about the median transapical plane, including a twisted, sigmoid central sternum. As a member of the monoraphid group, the frustules are heterovalvar and differ in ornamentation between the raphe valve and rapheless valve. Eucocconeis species occur commonly in the littoral zone of oligotrophic lakes, within epipelic, epipsammic, or epilithic habitats. Some species occur on wet walls.

Eunotia 42

  1. Valves with a short raphe
  2. Raphe extends from valve mantle onto valve face
  3. Striae uniserate
  4. One rimoportula usually present at valve apex

Eunotioid - Members of the Eunotiaceae (Eunotia, Actinella, Amphicampa) are unusual among the raphid diatoms in that frustules have a very short raphe system. The terminal nodules are positioned on the mantle. From the terminal nodule, the raphe slit lies on the valve mantle then slightly, or strongly, curves onto the valve face. As a result of this morphology, raphe branches visible in girdle view. Valves of Eunotia are...

Eupodiscus 1

  1. Areolae hexagonal
  2. Marginal ocelli present
  3. Marginal rimportulae present
  4. Margin with scalloped ridge

Centric - EUPODISCUS IS A MARINE TAXON Valves circular, with hexagonal, loculate areolae. The valve margin of the valve face possesses a scalloped, narrow ridge. Ocelli are present at the valve margins. The internal openings of the ocelli are depressed. Rimoportulae are present on the valve mantle, positioned between the ocelli. Eupodiscus is known from shallow, marine habitats.

Fallacia 5

  1. Valves linear-lanceolate to elliptical
  2. Hyaline area on valve face, characteristically lyre-shaped

Symmetrical biraphid - Valves of Fallacia are linear-lanceolate to elliptical in shape with bluntly rounded ends. A thin siliceous covering, or canopeum, covers the striae on the external valve surface. A lyre-shaped non-ornamented area is present on the valve surface. The canopeum can not be resolved in the light microscope, but slits, or linear openings of the canopeum, may be visible at the valve apices. In living cells, the canopeum may be...


  1. Frustules very lightly silicified
  2. Striae not visible with light microscopy
  3. Central sternum distinct

Symmetrical biraphid - The frustules of Fistulifera are very lightly silicified. Valves range in shape from linear to elliptical. Cells are small (usually < 8 µm) and possess numerous thin girdle bands, up to fourteen or more. Striae are not visible with light microscopy. The central sternum is distinct and may remain in a sample after the thin frustule has been destroyed in standard preparation. Fistulifera includes the former Navicula...

Fragilaria 8

  1. Frustules rectangular to lanceolate, in girdle view
  2. Frustules joined by spines to form colonies
  3. Single rimoportula present
  4. Girdle bands open

Araphid - Fragilaria frustules are rectangular to lanceolate in girdle view. The pattern of ornamentation on the valve face is variable, but a central sternum is generally present. Frustules are joined by small marginal spines to form ribbon like (band-shaped) colonies. A single rimoportula is present, usually positioned at a distal end. Small, apical porefields are also present. Girdle bands are open. Living cells contain...

Fragilariforma 5

  1. Central sternum very narrow, or absent
  2. Cells joined by spines to form colonies
  3. Striae uniseriate

Araphid - The valve margins of Fragilariforma are variable and may be linear, lanceolate, or elliptical with rostrate to capitate apices. In girdle view, frustules are rectangular in shape. Fragilariforma valves are characterized by a very narrow, or absent, central sternum. Striae are composed of distinct, uniseriate areolae. Frustules form linear colonies, linked by small marginal spines into ribbon like (band-shaped) colonies....

Frickea 1

  1. Distinct longitudinal ribs present
  2. Ribs extend to the helictoglossa, but do not fuse
  3. Rows of areolae present between ribs and raphe
  4. Large, T-shaped distal raphe fissures
  5. Large, T-shaped proximal raphe fissures

Symmetrical biraphid - Species within Frickea have valves that are linear-lanceolate with bluntly rounded apices. The striae are comprised of relatively small, round areolae. Internally, a set of longitudinal ribs encloses the full length of the raphe and is continuous through the center of the valve. The longitudinal ribs extend to a linear helictoglossa but do not fuse. Near the apices and central area, the areolae extend across the...

Frustulia 17

  1. Valve with distinct median longitudinal ribs
  2. Ribs form a single tip at valve ends
  3. Raphe present between longitudinal ribs
  4. Striae fine, punctate, forming transapical and longitudinal rows

Symmetrical biraphid - Frustulia valves are rhomboid to linear-lanceolate with straight to undulate margins. The striae are composed of fine areolae, arranged into patterns that appear to form both apical and transapical rows.The genus possesses distinct median, longitudinal ribs that extend most of the length of the valve. The raphe is located between the longitudinal ribs. At the valve terminus, the ribs form a single tip, or porte-crayon....

Geissleria 7

  1. Annulae present at poles
  2. Isolated punctum present (or absent) in the central area
  3. Valve elliptic or linear-elliptic, with rounded or rostrate ends

Symmetrical biraphid - Geissleria valves are elliptical to linear-elliptical, becoming obtuse, broadly rounded or rostrate at the ends. The striae are comprised of fine lineolate areolae; areolae are 50-80 in 10 µm. Distinctive annulae are present at the poles. The annulae are comprised of one to four transapical striae that interrupt the typical striae. The raphe is straight and filiform, with straight proximal ends. Terminal raphe ends are...


  1. Valve outline linear to linear-elliptic or linear-lanceolate
  2. Valve face-mantle junction smooth
  3. Mantle deep
  4. Striae uniseriate
  5. Areolae round to elliptic

Symmetrical biraphid - Genkalia was established in 2012 to accommodate Navicula digitulus and similar taxa. Valves are linear to linear-elliptic or linear-lanceolate in outline with rounded apices. The valve face-mantle junction is smooth and the mantle is deep. External proximal raphe ends are not expanded, but are often slightly curved towards primary valve side, or they may be straight. Terminal raphe fissures are long and broadly curved to...

Gliwiczia 1

  1. Frustules monoraphid
  2. Valves elliptic
  3. Striae and areolae more dense on raphe valve than on rapheless valve
  4. Internal hood on both valves
  5. Stauros on both valves

Monoraphid - Gliwiczia was established in 2013 to accommodate Achnanthes/Planothidium calcar and similar species. This genus has monoraphid frustules with a slightly concave raphe valve and almost flat, rapheless valve. Valves are elliptic. Both valves bear an internal hood or cavum and a stauros. Striae and areolae are considerably more dense and smaller on the raphe valve as compared to the rapheless valve. The raphe is straight,...

Gomphoneis 13

  1. Valves clavate, symmetric to the apical axis
  2. Pseudoseptae present
  3. Longitudinal lines may be present
  4. Striae multiseriate

Asymmetrical biraphid - Gomphoneis valves are clavate in both valve and girdle view. Valves are symmetrical to the apical axis. Two distinct groups are currently included within the genus Gomphoneis. One group contains species (including G. herculeana and G. eriense) with multiseriate striae crossed by longitudinal lines and with stigmata. The longitudinal lines are formed by either marginal lamellae or an axial plate, both internal structures...

Gomphonema 19

  1. Valve outline club-shaped
  2. Single stigma present, or absent
  3. Apical pore field present
  4. Striae uniseriate

Asymmetrical biraphid - Valves are symmetric to the apical axis, and asymmetric to the transapical axis. In valve view, valves are club-shaped, or clavate. In girdle view, frustules are wedge-shaped, or cuneate. A stigma may be present on one side of the central area. An apical pore field is present and bisected by the terminal raphe. Striae are uniseriate. Striae are not crossed by longitudinal lines, as in Gomphoneis. Many species grow on...

Gomphosinica 1

  1. Valves club-shaped
  2. Striae composed of 2-3 rows of areolae
  3. Stigma present
  4. Internally, stigma covered by hood
  5. Apical pore field present

Asymmetrical biraphid - Valves are clavate, or club-shaped, in valve view. In girdle view, frustules are cuneate. Striae are composed of 2-3 rows of areolae. Internally, the proximal raphe ends are unilaterally curved. A prominent stigma is present in the central area. Externally, the stigma has a round opening. Internally, the stigma has many small, round openings. An apical pore field is present at the footpole. The pore field has with...

Gomphosphenia 2

  1. Valves cuneate
  2. Axial area broad
  3. Proximal raphe with straight ends

Asymmetrical biraphid - Valves cuneate, with straight or undulate sides. Frustules are never bent or flexed. The raphe is straight and filiform, with straight external terminal fissures (both proximal and distal). Internally, the distal raphe fissure terminates in a helictoglossa. Stigmata are absent. Striae composed of open foramina, which may be obscured externally to give the appearance of ghost striae. Apical porefields, septa and...

Gyrosigma 2

  1. Valve sigmoid
  2. Raphe sigmoid
  3. Striae punctate, punctae form rows perpendicular and parallel to the axes

Symmetrical biraphid - Gyrosigma valves are sigmoid in valve outline. The axial area is narrow and also sigmoid, containing a sigmoid raphe. The striae are comprised of punctate areolae and form rows that are perpendicular and parallel to the apical and transapical axes. The central area is round to elliptical. External proximal raphe ends are deflected in opposite directions from one another. In living cells, two plastids are present....

Halamphora 8

  1. Raphe eccentric, positioned along ventral margin
  2. Dorsal margin of valve deeper than ventral margin
  3. Raphe ledge present on dorsal side of raphe
  4. Dorsal fascia usually absent

Asymmetrical biraphid - Valves are moderate, to strongly, asymmetric to the apical axis. Valves are symmetric to the transapical axis. The valve mantle is deep on the dorsal margin and shallow on the ventral margin. Most species of Halamphora lack a distinct marginal ridge; instead, the interface of the face and margin is gradual. A ‘raphe ledge’ is present on the dorsal side of the raphe. In most Halamphora species, a dorsal fascia, or hyaline...

Hannaea 1

  1. Valves arched
  2. Poles capitate to rostrate
  3. Valve with central ventral swelling
  4. Faint (ghost) striae near central swelling

Araphid - Valves are linear and arched. One or two rimoportulae occur on each valve, located near the valve ends. The ventral (concave) side of the valve has a central swelling. Faint, or ghost striae, are often present near the center of the valve. Hannaea is characteristic of oligotrophic streams and large lakes, where it grows attached in tufted colonies to benthic surfaces.


  1. Valves asymmetric to apical axis
  2. Raphe eccentric
  3. Frustule with both raphes on same side
  4. Striae uniseriate

Nitzschioid - Valves are asymmetric to the apical axis. Like Nitzschia, the raphe is contained within a canal and eccentrically positioned on the valve margin. In Hantzschia, the raphe is always located on the concave, or ventral, margin of the valve. Striae are uniseriate. The genus Hantzschia differs from Nitzschia in symmetry. Within a Hantzschia frustule, the raphes of the two valves are on the margin of the same side. In...


  1. Valves lanceolate with acute apices
  2. Central costae thickened, forming a stauos-like structure
  3. Raphe sternum with asymmetric thickened area in the central valve
  4. Striae crossed by longitudinal strips

Symmetrical biraphid - Valves are lanceolate with acute apices and a central raphe system. Two or three thickened transapical costae on either side of the valve center form a stauros-like structure. The raphe sternum is thickened on one side of the axial area and is fused to the central ribs, giving valves a cruciform appearance. Striae are uniseriate, containing rectangular or square poroids, and overlain externally by fine longitudinal...

Hippodonta 6

  1. Striae distinct, broad
  2. Striae uniseriate or biseriate
  3. Valve termini with thickened silica

Symmetrical biraphid - Species in Hippodonta possess thick, heavily silicified valves. Valves are generally lanceolate in valve outline with variously shaped poles. The striae are distinct, broad and may be uni-seriate or biseriate. The valve termini are thickened bands of silica.The raphe is straight and the proximal raphe ends are dilated. Two chloroplasts are present in living cells. Hippodonta includes the former Navicula capitata...

Humidophila 3

  1. Striae composed of single elongate areola
  2. External raphe ends curved or T-shaped

Symmetrical biraphid - Valves are small, usually less than 20 µm. Valves are linear, linear-elliptical to elliptical. The apices are broadly rounded. The central nodule is small and rounded. Other features of Humidophila are visible under SEM. The valve face is flat, and may have a narrow rim, or ridge, differentiating the valve face from the mantle. Striae on the valve face are composed of one usually transapically elongated, elliptical to...


  1. Valve margin appearing doubly triangular
  2. Porefields three, with ocelli present, positioned on the ends of one of the "triangles"

Centric - Hydrosera is a large multipolar, centric diatom. Frustules are heavily silicified. Valve margins appear to be in the shape of two superimposed triangles. Porefields, containing ocelli, are positioned at the three poles of one of the apparent triangles. Frustules form large zig-zag colonies, joined by mucilage secreted from the porefields. Hydrosera grows in streams of southern coastal areas and tropical islands. The...

Hygropetra 1

  1. Striae broad
  2. Striae alveolate
  3. Robust transapical costae

Symmetrical biraphid - The species in Hygropetra are small in size, with broad, V-shaped striae. The striae are alveolate, or composed of very small pores, in rows of 3-4. A small septum is present. Internally, robust transapical costae are positioned between the striae. The raphe is filiform, with a relatively large gap between proximal raphe ends. Hygropetra is a genus of small taxa previously considered within Pinnularia. The group is...

Karayevia 7

  1. Rapheless valve with nearly parallel striae
  2. Raphe valve with radial striae
  3. External distal raphe fissures curved to one side
  4. Internal expression of areolae occluded

Monoraphid - The valves of Karayevia are elliptic to lanceolate with rostrate to capitate ends. The rapheless valve possesses striae that are nearly parallel, composed of circular areolae. The raphe valve possesses radial striae, composed of transapically elongate areolae. Externally, the distal raphe fissures curve to the same side. Internally, the areolae are occulded. Cells grow attached to sand grains, with many species...

Kobayasiella 6

  1. Striae radiate, may be crossed by longitudinal lines
  2. Striae very fine
  3. Axial area narrow
  4. Raphe straight, proximal raphe ends expanded

Symmetrical biraphid - Kobayasiella valves are linear to linear-lanceolate, with ends rostrate or expanded and capitate. Because the striae often number more than 30 in 10 microns, they may not be visible in the light microscope. The striae are radiate in the center of the valve and become convergent at the valve apices. The raphe is filiform, with proximal raphe ends expanded. The raphe has fine, but distinctive ‘kinks’ or deflections at...

Krasskella 1

  1. Valves small
  2. Proximal raphe ends distant
  3. Striae composed of an elongate areola on valve face

Symmetrical biraphid - Valves are small and elongate. The axial and central areas are linear. Striae composed of an elongate areola on valve face and a circular areola on the valve mantle. The proximal raphe ends distant and the raphe fissures are slightly oblique. Helictoglossae at the proximal and distal raphe ends are not well developed. Frustules rectangular in girdle view. Krasskella currently includes only three species, K....

Kurtkrammeria 8

  1. Valves naviculoid, slightly dorsiventral
  2. Areolae slit-like or crescent shaped
  3. Striae convergent at the apices
  4. 1-3 dorsal stigmata present, or stigma absent
  5. Apical pore fields present or absent

Asymmetrical biraphid - Valves are naviculoid and slightly dorsiventral, symmetric to the transapical axis and asymmetric to the apical axis. The raphe is centrally located. Areolae in the striae are slit-like or crescent shaped and oriented with their long axes parallel to the apical axis. Transapical striae are radiate near the valve center, becoming parallel, then convergent near the apices. A more or less well developed apical pore field...

Lemnicola 1

  1. Apex narrowed, rounded and/or subrostrate
  2. Striae radiate
  3. Raphe valve with asymmetric stauros

Monoraphid - Valves of Lemnicola are linear to linear-elliptical. The valve apices may be narrowed, rounded, or subrostrate. The striae are slightly radial throughout and composed of biseriate rows of circular areolae. The raphe valve has an asymmetrical stauros. Lemnicola was erected for a single species, Lemnicola hungarica (Grun.) Round et Basson, which had previously been considered within Achnanthidium or Achnanthes. Living...

Lindavia 5

  1. One or more rimoportulae on valve face

Centric - Lindavia contains the taxa that were formerly included in the Cyclotella comta group, including species in the genera Puncticulata and Handmannia. One character that joins the lineage Lindavia is the presence of one or more rimoportulae on the valve face. That is, the rimoportulae are not positioned along side the marginal fultoportulae. Other characters of Lindavia include the type of satellite pores on the...

Luticola 2

  1. Valve linear-lanceolate to lanceolate with rounded, rostrate or capitate ends
  2. Stigma distinct
  3. Striae punctate
  4. Proximal raphe ends slightly deflected

Symmetrical biraphid - Valves of Luticola have an expanded central area with a distinct stigma in the central area. The striae are distinctly punctate. Proximal raphe ends are slightly deflected unilaterally. Distal raphe ends are deflected to the same side as the proximal raphe ends. Living cells possess a single chloroplast. Luticola is a widespread aerophilic genus, typical of soil and moss habitats. It includes the former Navicula mutica...

Mastogloia 7

  1. Valves elliptical to lanceolate, with rounded to capitate ends
  2. Valvocopula with partecta
  3. Striae punctate

Symmetrical biraphid - The valves of Mastogloia are elliptical to elliptic-lanceolate or lanceolate, with rounded to capitate ends. The valvocopula (first girdle band) each possess complex silica chambers, called partecta, that function to secrete strands of mucilage. The raphe is straight or undulate. The axial area is narrow, with a slightly expanded central area. Living cells have two plastids. Mastogloia is primarily a marine genus. A few...


  1. Cells small
  2. Axial area thickened
  3. Raphe straight

Symmetrical biraphid - Frustules of Mayamaea are small. The axial area is thickened and contains a straight raphe. Sometimes the axial area is all that remains from an acid preparation. The striae are uniseriate. Mayamaea includes the former Navicula atomus and its affiliates. Mayamaea species are found in ephemeral habitats as well as waters high in nutrient waste. Mayamaea species may reach high abundance in some polluted habitats.

Melosira 2

  1. Valve wall uniform in structure, little external ornamentation
  2. Frustules lack costae, septae, and spines
  3. Frustules may form colonies

Centric - Melosira valves have little ornamentation. They lack distinctive features including costae, septae, and spines. Frustules form long colonies linked at the valve face. Melosira contains a small number of freshwater species. M. varians, one of the most common species of the genus grows in benthic habitats of eutrophic streams and lakes. Melosira also contains the species M. undulata, which grows in large, oliogotrophic...

Meridion 5

  1. Valve symmetry to the transverse axis variable
  2. Frustules linear to wedge-shaped in girdle view
  3. Frustules may be joined in colonies
  4. Costae present

Araphid - Valves of Meridion are variable in symmetry to the transverse axis; they are either strongly heteropolar or isopolar. The heteropolar species are clavate (club-shaped) in valve view and wedge-shaped in girdle view. Septae (attached to girdle elements) are present. An apical porefield is present at one end. Valves may have 1-2 rimoportulae. Cells are joined together to form fan-shaped colonies. Chloroplasts are multiple...

Microcostatus 1

  1. Central sternum prominent
  2. Axial depressions present
  3. Axial depressions contain small costae

Symmetrical biraphid - Frustules of Microcostatus are small in size. The axial area includes lyre-shaped depressions on both sides of the prominent sternum. The depressions contain small costae (microcostae) in a depression near the axial area. Striae not visible with the light microscope. SEM reveals striae are present and comprised of single rows of areolae. In living cells two plastids are present, pressed to the cingulum. The genus differs...


  1. Striae composed of microfissures
  2. Frustules small
  3. Intermissio present

Symmetrical biraphid - SPECIES OF MICROFISSURATA ARE NOT KNOWN TO OCCUR IN NORTH AMERICA Species are comparatively small and usually live as solitary cells. Valves are lanceolate or linear elliptical, with protracted ends. The axial area is wide or very wide, especially in the central area. The central area is an expanded fascia. Externally, the striae are open slits, termed microfissures. Internally, the microfissures are closed and covered...

Muelleria 4

  1. External proximal raphe ends deflected to one side
  2. Longitudial canals two, one on each side of the raphe

Symmetrical biraphid - The outline of valves of Muelleria is linear, linear-elliptical or panduriform. The external proximal raphe ends are distinctive for their unilateral deflection. Distal raphe ends are branched. A longitudinal canal is present on each side of the raphe. Areolae are loculate. Living cells contain four plastids. Muelleria gibbula and M. terrestris appear to be widespread in aerophilic habitats, in association with soils,...

Navicula 84

  1. Striae composed of lineate areolae
  2. Sternum central, thickened

Symmetrical biraphid - Valves of Navicula tend to be elliptical to broadly-lanceolate in outline. Valve ends may be capitate, acute, rounded, or not expanded. The central sternum is thickened, and that thickening may be somewhat asymmetric. Pseudosepta may be present, or absent. The raphe is straight and filiform, or lateral in some species. The proximal raphe ends are slightly deflected to one side. The central area may be expanded, but the...

Navicymbula 1

  1. Asymmetry to apical axis slight
  2. Distal raphe ends dorsally deflected
  3. Striae lineate

Asymmetrical biraphid - The valve outline is very slightly asymmetric to the apical axis, valves are nearly naviculoid in appearance. The raphe is positioned in a nearly central position on valve. The distal raphe ends are deflected dorsally. Striae are lineate. Valves lack apical pore fields. The genus is more closely allied with Navicula than Cymbella. Navicymbula pusilla, one of the brackish to saline taxa, is found in endorheic lakes of...