Visit us at

Diatoms of the United States is now known as Diatoms of North America.
This site has been redesigned and updated, and will be closing soon.

GENERA starting with "a"


Acanthoceras 1

  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....

Adlafia 1

  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...

Amphicampa 1

  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 6

  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 5

  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 2

  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....