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


Symmetrical biraphid - Valves are rhomboid or bluntly lanceolate. The central area is variable in shape, from a large fascia-like central area to small and oval. The raphe is straight, with curved distal fissures. Frustules possess numerous girdle bands. Two chloroplasts with deep lobes are present in living cells. This genus includes primarily marine species, with few freshwater species. Some marine littoral species create mucilage tubes.

Eunotioid - Peronia is both heteropolar and heterovalvar. Valves have a narrow, rounded footpole and a wider headpole. The headpole is rounded to subcapitate, while the footpole is narrow and rounded. Valves are heterovalvar in that one valve has a raphe that extends from each pole to approximately one quarter of the valve length. The other valve has a rudimentary raphe that may be evident at the poles, or may be completely absent....

Pinnularia 18

  1. Striae chambered, may form longitudinal lines
  2. External proximal raphe ends dilated, bent slightly
  3. Distal raphe ends deflected

Symmetrical biraphid - Frustules of Pinnularia may be large, up to 300 µm in length. The striae are alveolate. Internally, the striae are positioned within chambers. The openings of the chambers are evident as longitudinal lines that cross the striae. The raphe system may be straight or complex. Externally, the proximal raphe ends are expanded and bent slightly to the same side. The distal raphe ends are deflected and may form a distinct,...

Placoneis 5

  1. Valve linear to lanceolate
  2. Apices rostrate to capitate
  3. Striae uniserate
  4. Stigmata present or absent

Symmetrical biraphid - The valve outline of Placoneis is linear to lanceolate, sometimes with rostrate or capitate ends. The areolae are loculate and internally occluded. Areolae form uniseriate striae. The axial area is narrow. The proximal raphe ends are straight and slightly expanded externally. The plastid is large, distinct, and divided into two X-shaped plates. Placoneis contains a number of species that grow in epipelic habitats of...

Plagiotropis 2

  1. Valves lanceolate, poles narrow to pointed
  2. Valve face folded, forming prominent apical "lines" on either side of the raphe
  3. Raphe raised and positioned within a keel

Symmetrical biraphid - The valve margins of Plagiotropis are lanceolate with narrow poles. The valve face is folded, with folds appearing as lines on either side of the raphe. The raphe itself is raised above the valve face and positioned within a keel. The axial area is narrow and the central area is variable in shape. Striae are parallel and composed of loculate areolae. In girdle view, cells appear constricted in the center. Plagiotropis...

Planothidium 10

  1. Frustules heterovalvar - one valve with a raphe, the other lacking a raphe
  2. Striae multiseriate
  3. Terminal raphe fissures long, curved to secondary side

Monoraphid - Frustules of Planothidium are heterovalvar, with slightly concave raphe valve and convex rapheless valve. Valves are elliptic to lanceolate, with rounded, rostrate or capitate apices. The striae are multiseriate. The raphe valve possess radiate striae and a prominent raphe. The terminal raphe fissures are long, curved and deflected unilaterally. Many Planothidium species, but not all, possess an asymmetric central area...

Platessa 7

  1. Frustules heterovalvar - one valve with a raphe, the other lacking a raphe
  2. Valves nearly flat
  3. Valves elliptic, linear-elliptic to elliptic-lanceolate
  4. Striae mostly biseriate, sometimes uniseriate on raphe valve
  5. Terminal raphe fissures absent

Monoraphid - Frustules are heterovalvar, one valve possesses a raphe and the other valve lacks a raphe. Valves are elliptic, linear-elliptic or elliptic-lanceolate. Both of the valve faces are nearly flat, in contrast to other monoraphids that have convex and concave valve faces. Striae are mostly biseriate, but in some species the raphe valve face may be uniseriate. Terminal raphe fissures are absent. Instead, the raphe ends with...

Playaensis 1

  1. Raphe bordered by prominent longitudinal rib
  2. Valves panduriform with marginal spines
  3. Valves less than 40 µm in length

Symmetrical biraphid - Frustules of Playaensis are small in size, less than 40 µm in length. Valves are panduriform in shape and the valve margins are ornamented with small, fin-like spines. Striae are uniseriate and punctate. The raphe is bordered on both sides by a longitudinal rib. Internally, this rib is visible as a broad structure. Externally, the surface of valves are ornamented with spherical nodules. Playaensis is known from...

Pleurosigma 1

  1. Valves sigmoid
  2. Raphe sigmoid
  3. Striae decussate

Symmetrical biraphid - Valves are elongate and sigmoid. Striae are composed of evenly spaced decussate rows. The rows are arranged in 3 patterns: a trans-apical row and two opposing oblique rows. Internally, the areolae are occluded by hymenes. The raphe is strongly sigmoid, with a narrow axial area. The sternum is thickened equally on both sides. The raphe terminates proximally in a small oval, expanded central area defined by two curved...

Pleurosira 1

  1. Valves orbicular
  2. Ocelli 2-4, prominent
  3. Rimoportula single, positioned near center of valve
  4. Valves form zig-zag colonies

Centric - Pleurosira has very large orbicular valves with 2-4 prominent ocelli, the number depending on salinity. The ocelli secret mucopolysaccaride pads that join cells together in large, zig-zag colonies. A single rimoportula is positioned near the center of the valve face. Colonies form filaments that are visible in streams. Pleurosira is widely distributed in naturally saline waters and polluted waters. In older literature...


  1. Valve face transversely undulate
  2. Rows of areolae radiate from valve center
  3. Valve face with central fultoportulae

Centric - The valve face of Pliocaenicus has a distinct transverse undulation. Areolae radiate from the valve center in single rows, rather than being organized into groups, or fascicles. A number of fultoportulae are positioned at the center of valve face. Marginal fultoportulae are present between the striae, near the valve/mantle interface. Species in Pliocaenicus lack areolae on the valve mantle, below the marginal strutted...

Prestauroneis 3

  1. Central striae radiate
  2. Central striae widely spaced
  3. Pseudoseptae present

Symmetrical biraphid - Valves are lanceolate to lanceolate-elliptical. The raphe extends the length of the valve, onto the valve mantle. Pseudosepta are present at the apices. The striae are radial at the center of the valve, becoming nearly parallel at the apices. The striae are uniseriate and composed of small, round or elliptical areolae. At the center of the valve, striae are separated by robust interstriae (virgae). The external proximal...

Psammothidium 17

  1. Frustules heterovalvar - one valve with a raphe, the other lacking a raphe
  2. Raphe valve convex, rapheless valve concave
  3. Valves linear, elliptic or lanceolate
  4. Striae usually uniseriate
  5. Terminal raphe fissures may curve to opposite sides, or absent

Monoraphid - Frustules are small, elliptic, lanceolate-elliptic, or linear-elliptic in shape. Frustules are bent in girdle view so that the raphe valve is convex and the rapheless is concave, an arrangement that differs from other achnanthoid genera. The length to width ratio ranges between 1.5 and 3.0. The valve apices are broadly rounded. The arrangement of areolae is similar on the raphe and rapheless valves. Striae are radiate...

Pseudofallacia 2

  1. Cells small
  2. Stria compose of single elongated areola
  3. Conopeum present, finely perforated

Symmetrical biraphid - The genus Pseudofallacia was established in 2012 for small, solitary naviculoid species that had been included in Fallacia. Valves are symmetric with respect to the apical and transapical axes. The raphe is filiform and straight. A finely perforated conopeum is present, which extends along the raphe. Two pores are present at each terminal area of the valve. A single elongated areola makes up each stria. Externally, the...

Pseudostaurosira 8

  1. Striae uniseriate
  2. Spines present
  3. Areolae elliptical

Araphid - Frustules of Pseudostaurosira form chains with frustules linked valve face to valve face. Frustules rectangular in girdle view. Striae are uniseriate, composed of a few (usually not more than 4) elliptical areolae. Spines are present on the valve margin and may be branched. Pseudostaurosira includes the former Fragilaria brevistriata and is distinguished by the structure of the striae, rather than the shape of the valve...


  1. Striae uniseriate, radial
  2. Apical porefield present at each pole
  3. Valve margin spinose
  4. Spines flattened, bifurcate

Araphid - Pseudostaurosiropsis includes species that are small, with elliptic to rounded valves. The striae are uniseriate, composed of 2-6 round areolae externally occluded with discoid volae. Two apical porefields are present, composed of a few, isolated pores. Spines are present at the valve margin, aligned with a stria. The spines are flattened and bifurcate. Valves lack rimoportulae. To date, species have been described...

Punctastriata 1

  1. Striae composed of net-like areolae
  2. Spines present

Araphid - Valves linear elliptical and may possess a central inflation. The striae are composed of a rectangular net of transapical and apical bars. Spines are present at the valve face/mantle junction on the costae, interrupting the striae, or both positions. The spines are variable in form. According to Williams and Round (1987) one apical porefield is present; according to Morales (2005) two apical porefields are present. The...


  1. Central area flat or concentrically undulate
  2. Valve face divided into two parts, each with distinct morphology

Centric - THE GENUS PUNCTICULATA, ALTHOUGH VALIDLY PUBLISHED, IS NOT LEGITIMATE. TAXA HAVE BEEN TRANSFERRED TO LINDAVIA. THIS PAGE WILL SOON BE CLOSED. The valve face is circular or oval. The central area is variable: with both areolae and fultoportulae, or areolae only, or fultoportulae only. Central area fultoportulae, when present, possess three satellite pores. One or more rimoportulae are present on the valve face, at end...