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Diatoms of the United States is now known as Diatoms of North America.
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GENERA starting with "s"


Scoliopleura 1

  1. Valves twisted about apical axis
  2. Longitudinal canals evident on both sides of the raphe
  3. Proximal raphe ends deflected in opposite directions

Symmetrical biraphid - Although the valve margins of Scoliopleura are linear-lanceolate, the frustules are twisted about the apical axis. Longitudinal canals are present on both sides of the raphe. The raphe is slightly sigmoid because of the twist, or torsion, of the frustule. The proximal raphe ends are deflected in opposite directions and the distal raphe ends are divergent. Striae are formed by loculate punctae. A single species is found...

Sellaphora 20

  1. Valve linear, lanceolate or elliptic
  2. Polar bars at apices, in some taxa
  3. Striae uniseriate or biseriate
  4. Conopeum present, in some taxa

Symmetrical biraphid - The valve is linear, lanceolate, or elliptic with bluntly rounded poles. Polar bars, or transapical thickenings, are present in some taxa. The axial area is distinct, and may be expanded along the apical axis to form a conopeum. The external proximal raphe ends are dilated, while the distal raphe ends are deflected. Sellaphora is widespread in alkaline to brackish waters of neutral pH. It contains the former Navicula...

Semiorbis 2

  1. Valves strongly lunate
  2. Valve face with transapical external costae
  3. Spines present on the dorsal and ventral ends of costae

Eunotioid - Valves are strongly lunate, with thick, external, transverse costae between striae. The costae often terminate in spines at the dorsal and ventral margins. A small, greatly reduced raphe is present at the poles. The raphe is typically not visible in LM. The proximal raphe ends are on the ventral margin, while the terminal fissures curve slightly onto the valve face. Internally, the terminal raphe ends in a helictoglossa....

Simonsenia 1

  1. Raphe within a raised canal
  2. Raphe positioned on valve margin
  3. Valve with regularly spaced depressions near margin

Nitzschioid - Simonsenia is a genus that possesses characters seen in both the Surirellaceae and the Nitzschiaceae. Its valve symmetry and general shape, with the raphe being present on only one valve margin, resembles that of many Nitzschia species. Also, the cribra of the areolae are similar to those in many Nitzschia species. However, its raphe is present in a canal that is raised above the valve surface, as in some species of...


  1. Valves lightly silicified
  2. Elongate tubular process links cells to form colonies

Centric - Valves of Skeletonema are very lightly silicified and often collapse when dried, or dissolve with standard processing methods. The marginal fultoportulae are positioned at the base of each connecting or tubular process. This process links frustules into long colonies. A single rimoportula is located at the base of the connecting process. Skeletonema grows primarily in marine and estuaries. Species within the genus are...

Spicaticribra 1

  1. Valve face flat to slightly domed
  2. Central areolae may be larger than marginal areolae
  3. Marginal fultoportulae present
  4. Areolae coarse

Centric - Spicaticribra has a flat, to slightly domed valve face, with a sloping mantle. Marginal fultoportulae are present and 1-3 rimportula. Striae are coarse, radial and straight, with some striae extending from the central valve to the margin, others extending only partially across the valve. Species in the genus include S. kingstonii Johansen et al., S. kodaikanaliana Karthick and Kociolek and S. rudis (Tremarin et al.)...

Stauroforma 1

  1. Valves lack rimoportulae
  2. Striae meet in central valve face
  3. Central area absent

Araphid - Frustules of Stauroforma are small, with an elliptic to lanceolate valve outline. The valves not centrally expanded. Rimoportulae are absent in species of Stauroforma. The striae are continuous across the valve face, although this feature may be difficult to resolve in the light microscope. Frustules are linked together into linear colonies. Spines may be present or absent. Stauroforma is composed of species formerly...

Stauroneis 48

  1. Stauros present
  2. Pseudosepta may be present
  3. Striae distinctly punctate

Symmetrical biraphid - Stauroneis has naviculoid and mostly solitary cells with two chloroplasts, one on each side of the cell against the cingulum. Valves range from almost linear to lanceolate to elliptic-lanceolate. The central area is a prominent transverse fascia, known as a “stauros”. The central fascia typically extends to the valve margins, where one to several short striae may be present. The striae are uniseriate and punctate....

Staurophora 6

  1. Fascia with short striae at margins
  2. Deep, highly arched valves
  3. Striae composed of small round poroids
  4. Single lobed chloroplast with large pyrenoid

Symmetrical biraphid - Valves of Staurophora are solitary and lanceolate, linear-lanceolate, or elliptic-lanceolate, often with protracted ends. Valve faces are strongly curved onto the mantle, which is relatively deep. As a result, cleaned frustules often come to rest in girdle view. A fascia is present and is interrupted at the margins by several short striae. These short striae may not be visible in valve view. Striae are composed of small...

Staurosira 4

  1. Striae narrow, composed of small, round or somewhat elongated areolae
  2. Striae do not meet in central sternum

Araphid - Staurosira valves are elliptical or cruciform in outline. In girdle view, the frustules are rectangular in shape. Cells often form colonies, joined by interlinking spines. The striae are narrow and composed of small, round areolae. Striae do not meet one another in the central sternum. Valves lack rimoportulae. Staurosira includes the former Fragilaria construens and its relatives. In the light microscope it can be...

Staurosirella 5

  1. Striae composed of lineolate areolae
  2. One to two apical porefields
  3. Valves oval, linear or cruciform

Araphid - The valves of Staurosirella are oval, linear or cruciform. Striae are composed of lineolate areolae. One to two apical porefields are present. Rimoportulae are absent. Neighboring cells may be linked together at valve faces to form colonies. Staurosirella includes the former Fragilaria lapponica, F. pinnata, F. leptostauron and their allies. Cells attach to substrates by short stalks secreted by the apical porefield....


  1. Valves long, thin
  2. Valves lanceolate or sigmoid
  3. Raphe within a canal
  4. Raphe located along entire valve margin

Surirelloid - The valves of Stenopterobia are narrow and may be straight or sigmoid in outline. Like other members of the Surirellaceae, the raphe is located within a canal along the valve margin. The canal in Stenopterobia is raised above the valve, onto a keel. Species within Stenopterobia are typically found in low conductivity, nutrient poor, acidic waters. For example, Stenopterobia often dominates the pond habitats of the...


  1. Cell wall comprised of a single layer
  2. Marginal chambers absent

Centric - Stephanocyclus differs from Cyclotella by the absence of a two-layered valve wall. Stephanocyclus differs from Cyclostephanos by the absence of marginal chambers. Stephanocyclus contains species that were included with Cyclotella meneghiniana and its allies, including C. caspia, and C. quillensis. Many Stephanocyclus species, however, have not been formally transferred. Stephanocyclus species are widely distributed in...

Stephanodiscus 10

  1. Striae radial and organized into fascicles
  2. Spines marginal
  3. Valve face flat or concentrically undulate
  4. Marginal spines may be subtended by marginal fultoportulae

Centric - The valve face of Stephanodiscus species is flat or concentrically undulate.The striae are radial and organized into bundles, or fascicles. Fascicles are separated from one another by costae. Spines are present on the valve margin and a marginal fultoportula may be positioned below the spines. Internally, the areolae are covered by domed cribra. Stephanodiscus is a diverse and widespread planktonic genus of freshwaters,...

Surirella 14

  1. Frustules isopolar or heteropolar
  2. Raphe positioned along entire valve margin
  3. Raphe located within a canal, which may be raised above valve surface

Surirelloid - Cells of Surirella grow as single, isolated cells. The frustules may be either isopolar or heteropolar. Like other genera with in the family Surirellaceae, the raphe system is positioned along the margin of the valve. The raphe is located within a canal, which may be raised above the valve surface in some species. Cells may be highly silicified, with spines and silica nodules on the valve surface. Species in Surirella...

Synedra 4

  1. Vales linear to lanceolate with rounded ends
  2. Striae parallel
  3. Apical porefield present at each pole

Araphid - Currently, species in Synedra are considered to be primarily marine, typified by S. balthica, which is synonomous with Synedra gaillonii (Bory) Ehrenberg. The taxon shown here is S. gaillonii. Following recent changes in the literature, elongate, needle-like freshwater species on the Diatoms of the United States site are treated in Fragilaria and Ulnaria. Araphid genera, including Synedra, Fragilaria, Ulnaria and...