The overall classification of diatoms is proposed by Round et al. (1990), which recognised three classes: the Coscinodiscophyceae, the Fragilariophyceae, and the Bacillariophyceae. During the last 10 years, molecular genetic studies have shown that this classification does not accurately reflect phylogeny. For example, the centric diatoms, which were all classified in the Coscinodiscophyceae by Round et al., have proved to be ‘paraphyletic' with respect to the pennate diatoms, which Round et al. (2004) put in two classes, the Fragilariophyceae (araphid pennate diatoms) and the Bacillariophyceae (raphid pennate diatoms). In other words, the ancestor of the two pennate diatom groups was not also the ancestor of the centric diatoms; instead, it was a centric diatom belonging to just one of many centric diatom lineages. However, despite much research, diatomists have yet to agree on a satisfactory replacement for the Round et al (1990).

Classes within Bacillariophyta

Fragilariophyceae Round in Round et al. (1990)

Cells elongate and usually symmetrical about at least two planes, one of which is the pervalvar plane. Stria pattern bilateral, orientated about a longitudinal rib (the sternum), as in a feather, but raphe always absent. Costae and septa may be present. Rimoportules usually present. Many genera within the class have many small, discoid plastids lying under the valve face, but some have one, two or a few plastids. Many genera have rows of small spines at the junction of the valve face and mantle, which allow sibling cells to form filamentous colonies. However, taxa also form zig-zag or stellate colonies, where cells are joined at the base by mucous secreted from apical pore fields, or clusters radiating from a common pad or stalk.

Bacillariophyceae Haeckel 1878 emend. D.G. Mann in Round et al. (1990)

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.

Coscinodiscophyceae Round et R.M. Crawford in Round et al. (1990)

Cells circular, triangular or quadrate, or elongate, usually symmetrical about at least two planes, one of which is the pervalvar plane. Stria pattern radial, orientated about a small boss or ring. Raphe always absent and costae and septa very rare. Rimoportules usually present. Mostly with many small discoid plastids. Sometimes connected into filamentous or zig-zag colonies by spines, pads of mucous or chitin threads.

Taxonomic position of Diatom genera (Based on the classification system of Round et al., 1990.)

Kingdom: Plantae

Subkingdom: Chromista

Division: Bacillariophyta

Class: COSCINODISCOPHYCEAE Round & Crawford (CENTRIC DIATOM)

Subclass: Thalassiosirophycidae Round & Crawford

Order: Thalassiosirales Glezer & Makarova 1986

Family: Thalassiosiraceae Lebour 1930

Genus: Thalassiosira, Planktoniella, Porosira, Minidiscus, Bacteriosira

Family: Skeletonemataceae Lebour 1930

Genus: Skeletonema, Detonila

Family: Stephanodiscaceae Glezer & Makarova 1986

Genus: Cyclotella, Cyclostephanos, Stephanodiscus, Mesodictyon, Pleurocyclus, Stephanocostis

Family: Lauderiaceae (Schutt) Lemmermann 1899

Genus: Lauderia

Subclass: Coscinodiscophycidae Round & Crawford

Order: Chrysanthemodiscales Round

Family: Chrysanthemodiscaceae Round 1978

Order: Melosirales Crawford

Family: Melosiraceae Kutzing 1844

Genus: Melosira, Druridgea

Family: Stephanopyxidaceae Nikolaev

Genus: Stephanopyxis

Family: Endictyaceae Crawford

Genus: Endictya

Family: Hyalodiscaceae Crawford

Genus: Hyalodiscus

Order: Paraliales Crawford

Family: Paraliaceae Crawford, 1988

Genus: Paralia, Ellerbeckia

Order: Aulacoseirales Crawford

Family: Aulacoseiraceae Crawford

Genus: Aulacoseria, Strangulonema

Order: Orthoseirales Crawford

Family: Orthoseiraceae Crawford

Genus: Orthoseira

Order: Coscinodiscales Round & Crawford

Family: Coscinodiscaceae Kutzing 1844

Genus: Coscinodiscus, Palmeria, Stellarima, Brightwellia, Craspedodiscus

Family: Rocellaceae Round & Crawford

Genus: Racella

Family: Aulacodiscaceae (Schutt) Lemmermann 1903

Genus: Aulacodiscus

Family: Gossleriellaceae Round

Genus: Gossleriella

Family: Hemidiscaceae Hendey 1937 emend Simonsen 1975

Genus: Hemidiscus, Actinocyclus, Azpeitia, Roperia

Family: Heliopeltaceae H. L. Smith 1872

Genus: Actinoptychus, Glorioptychus, Lepidodiscus

Order: Ethmodiscales Round

Family: Ethmodiscaceae Round

Genus: Ethmodiscaceae

Order: Stictocyclales Round

Family: Strictocyclaceae Round 1978

Genus: Strictocyclus

Order: Asterolamprales Round and Crawford

Family: Asterolampraceae H. L. Smith 1872

Genus: Asterolampra, Asteromphalus

Order: Arachnoidiscales Round

Family: Arachnoidiscaceae Round

Genus: Arachnoidiscus

Order: Stictodiscales Round and Crawford

Family: Stictodisceae (Schutt) Simonsen 1972

Genus: Stictodiscus

Subclass: Biddulphiophycidae Round & Crawford

Order: Triceratiales Round and Crawford

Family: Triceratiaceae (Schutt) Lemmermann 1899

Genus: Triceratium, Odontella, Lampriscus, Sheshukovia, Pseudoauliscus, Eupodiscus, Pleuosira, Amphitetras, Cerataulus, Auliscus

Family: Plagiogrammaceae De Toni 1890

Genus: Palgiogramma, Glyphodesmis, Dimeregramma, Dimeregrammopsis

Order: Biddulphiales Krieger 1954

Family: Biddulphiaceae Kutzing 1844

Genus: Biddulphiaceae, Biddulphiopsis, Hydrosera, Isthmia, Trigonium, Tersinoe, Pseudotriceratium

Order: Hemiaulales Round & Crawford

Family: Hemiaulaceae Heiberg 1844

Genus: Hemiaulus, Eucampia, Climacodium, Cerataulina, Trinacria, Abas, Briggera, Pseudorutilaria, Keratophora, Kittonia, Strelnikovia, Riedelia, Baxteriopsis, Sphynctolethus, Ailuretta

Family: Bellerocheaceae Crawford

Genus: Bellerochea, Subsilicea

Family: Streptothecaceae Crawford

Genus: Streptotheca, Neostreptotheca

Order: Anaulales Round & Crawford

Family: Anaulaceae (Schutt) Lemmermann 1899

Genus: Anaulus, Eunotogramma, Porpeia

Subclass: Lithodesmiophycidae Round & Crawford

Order: Lithodesmiales Round & Crawford

Family: Lithodesmiaceae Round

Genus: Lithodesmium, Lithodesmioides, Ditylum

Subclass: Corethrophycidae Round & Crawford

Order: Corethrales Round & Crawford

Family: Corethraceae Lebour 1930

Genus: Corethron

Subclass: Cymatosirophycidae Round & Crawford

Order: Cymatosirales Round & Crawford

Family: Cymtosiraceae Hasle. Von Stosch & Syvertsen 1983

Genus: Cymatosira, Campylosira, Plagiogrammopsis, Brockmanniella, Minutocellus, Leyanella, Arcocellus, Papiliocellulus, Extubocellulus

Family: Rutilariaceae De Toni 1894

Genus: Rutilaria, Syndetocystis

Subclass: Rhizosoleniophycidae Round & Crawford

Order: Rhizosoleniales Silva 1962

Family: Rhizosoleniaceae De Toni 1890

Genus: Rhizosolenia, Proboscia, Pseudisolenia, Urosolenia, Guinardia, Dactyliosolen

Family: Pyxillaceae (Schutt) Simonsen 1972

Genus: Pyxilla, Gladius, Gyrodiscus, Mastogonia, Pyrgupyxis

Subclass: Chaetocerotophycidae Round & Crawford

Order: Chaetocerotales Round & Crawford

Family: Chaetocerataceae Ralfs in Pritchard 1861

Genus: Chaetoceros, Gonioceros, Bacteriastrum

Family: Acantocerataceae Crawford

Genus: Acanthoceros

Family: Attheyaceae Round & Crawford

Genus: Attheya

Order:s: Leptocylindrales Round & Crawford

Family: Leptocylindraceae Lebour 1930

Genus: Leptocylindrus

Class: FRAGlLARIOPHYCEAE Round (ARAPHID, PENNATE DIATOM)

Subclass: Fragilariophycidae Round

Order: Fragilariales Silva 1962 sensu emend

Family: Fragilariaceae Greville 1833

Genus: Fragilaria, Centronella, Asterionella, Staurosirella,  Staurosira, Pseudostaurosira, Punctastriata, Fragilariaforma, Martyana, Diatoma, Hannaea, Meridion, Synedra, Ctenophora, Neosynedra, Tabularia, Catacombas, Hyalosynedra, Opephora, Trachysphenia, Thalassioneis, Falcula, Pteroncola, Asterionellopsis, Bleakeleya, Podocystis

Order: Tabellariales Round

Family:Tabellariaceae Kutzing 1844

Genus: Tabellaria, Tetracyclus, Oxyneis

Order: Licmophorales Round

Family: Licomophoraceae Kutzing 1844

Genus: Licmophora, Licmosphenia

Order: Rahphoneidales Round

Family: Rhaphoneidaceae Forti 1912

Genus: Rhaphoneis, Diplomenora, Delphineis, Neodelphineis, Perissonoe, Sceptroneis

Family: Psammodiscaceae Round & Mann

Genus: Psammodiscus

Order: Ardissoneales Round

Family: Ardissoneaceae Round

Genus: Ardissonea

Order: Toxariales Round

Family: Toxariaceae Round

Genus: Toxarium

Order: Thalassionematales Round

Family: Thalassionemataceae Round

Genus: Thalassionema, Thalassiothrix, Trichotoxon

Order: Rhabdonematales Round & Crawford

Family: Rhabdonemataceae Round & Crawford

Genus: Rhabdonema

Order: Striatellales Round

Family: Striatellaceae Kutzing 1844

Genus: Striatella, Microtabella, Grammatophora

Order: Cyclophorales Round & Crawford

Family: Cyclophoraceae Round & Crawford

Genus: Cyclophora

Family: Entopylaceae Grunow 1862

Genus: Entopyla, Gephyria

Order: Climacospheniales Round

Family: Climacospheniaceae Round

Genus: Climcosphenia, Synedrasphenia

Order: Protoraphidales Round

Family: Protoraphidaceae Simonsen 1970

Genus: Protoraphis, Pseudohimantidium

Class: BACILLARIOPHYCEAE Haeckel 1878 (RAPHID, PENNATE DIATOM)

Subclass: Eunotiaphycidae D. G Mann

Order: Eunotiales Silva 1962

Family: Eunotiaceae Kutzing 1844

Genus: Eunotia, Actinella, Semiorbis, Desmogonium

Family: Peroniaceae (Karsten) Topachevs'kyj & Oksiyuk 1960

Genus: Peronia

Subclass: Bacillariophycidae D. G. Mann

Order: Lyrellales D. G. Mann

Family: Lyrellaceae D. G. Mann

Genus: Lyrella, Petroneis

Order: Mastogloiales D. G. Mann

Family: Mastogloiaceae Mereschkowsky 1903

Genus: Aneumastus, Mastogloia

Order: Dictyoneidales D. G. Mann

Family: Dictyoneidales D. G. Mann

Genus: Dictyoneis

Order: Cymbellales D. G. Mann

Family: Rhoicospheniaceae Chen & Zhu 1983

Genus: Rhoicosphenia, Campylopyxis, Cuneolus, Gomphoseptatum

Family: Anomoeoneidaceae D. G. Mann

Genus: Anomoeoneis, Staurophora

Family: Cymbellaceae Greville 1833

Genus: Plaaconeis, Cymbella, Brebissonis, Encyonema, Gomphocymbella

Family: Gomphonemataceae Kutzing 1844

Genus: Gomphonema, Didymosphenia, Gomphoneis, Reimeria, Gomphopleura

Order: Achnanthales Silva 1962

Family: Achnanthaceae Kutzing 1844

Genus: Achnanthes

Family: Cocconeidaceae Kutzing 1844

Genus: Cocconeis, Camphyloneis, Anothoneis, Bennettella, Epipellis

Family: Achnanthidiaceae D. G. Mann

Genus: Achnanthidium, Eucocconeis

Order: Naviculales Bessey 1907 sensu emend.

Suborder: Neidiineae D. G. Mann

Family: Berkeleyaceae D. G. Mann

Genus: Parlibellus, Berkeleya, Climaconeis, Stenoneis

Family: Cavinulaceae D. G. Mann

Genus: Cavinula

Family: Cosmioneidaceae D. G. Mann

Genus: Cosmioneis

Family: Scolioneidaceae D. G. Mann

Genus: Scolioneis

Family: Diadesmidaceae D. G. Mann

Genus: Diadesmis Luticola,

Family: Amphipleuraceae Grunow 1862

Genus: frickea, Amphipleura, Frustulia, Cistula

Family: Brachysiraceae D. G. Mann

Genus: Brachysira

Family: Neidiaceae Mereschkowsky 1903

Genus: Neidium

Family: Scoliotropidaceae Mereschkowsky 1903

Genus: Scoliopleura, Scoliotropics, Biremis, Progonoia, Diadema

Suborder: Sellaphorineae D. G. Mann

Family: Sellaphoraceae Mereschkowsky 1902

Genus: Sellaphora, Fallacia, Rossia, Caponea

Family: Pinnulariaceae D. G. Mann

Genus: Pinnularia, Diatomella, Ostrupia, Dimidiata

Suborder: Phaeodactylineate J. Lewin 1958

Family: Phaeodactylaceae Silva 1962

Genus: Phaeodactylum

Suborder: Diploneidineae D. G. Mann

Family: Diploneidaceae D. G. Mann

Genus: Diploneis, ? Raphidodiscus

Suborder: Naviculineae Hendey 1937

Family: Naviculaceae Kutzing 1844

Genus: Navicula, Trachyneis, Pseudogomphonema, Seminavis, Rhoikoneis, Haslea, Cymantoneis

Family: Pleurosigmataceae Mereschkowsky 1903

Genus: Pleurosigma Toxonidea, Donkinia, Gyrosigma, Rhoicosigma

Family: Stauroneidaceae D. G. Mann

Genus: Plagiotropis, ?Stauropsis, Pachyneis

Family: Proschkiniaceae D. G. Mann

Genus: Proschkinia

Order: Thalassiophysales D. G. Mann

Family: Catenulaceae Mereschkowsky 1902

Genus: Catenula, Amphora, Undatella

Family: Thalassiophyceae D. G. Mann

Genus: Thalassiophysa

Order: Bacillariales Hendey 1937

Family: Bacillariceae Ehrenberg 1831

Genus: Bacillaria, Hantzschia, Psammodictyon, Tryblionella, Cymbellonitzschia, Gomphonitzschia, Gomphotheca, Nitzschia, Denticula, Denticulopsis, Fragilariopsis, Cylindrotheca, Simonsenia, Cymatonitzschia, Perrya

Order: Rhopalodiales D. G. Mann

Family: Rhopalodiaceae (Karsten) Topachevs'kyj & Oksiyuk 1960

Genus: Epithemia, Rhopalodia, Protokeelia

Order: Surirellales D. G. Mann

Family: Entomoneidaceae Reimer in Patrick & Reimer 1975

Genus: Entomoneis

Family: Auriculaceae Hendey 1964

Genus: Auricula

Family: Surirellaceae Kutzing 1844

Genus: Hydrosilicon, Petrodisctyon, Plagiodiscus, Stenopterobia, Surirella, campylodiscus, Cymatopleura