• Aaptotoichus challengeri Holbourn & Kaminski, 1995 †
• Aaptotoichus clavellatus (Loeblich & Tappan, 1946) †
• Aaptotoichus mutinensis Haig & McCartain, 2010 †
• Aaptotoichus quinnaniensis Haig, 2003 †
• Aaptotoichus vallis (Trifonova, 1967) †
• Abadehella biconvexa Okimura & Ishii, 1975 †
• Abadehella coniformis Okimura & Ishii, 1975 †
• Abadehella grandis Pronina-Nestell, 2001 †
• Abadehella iwaiensis Kobayashi, 1997 †
• Abadehella koryakiensis Daydov in Davydov et al., 1996 †
• Abadehella longlinensis Lin, Li & Sun, 1990 † accepted as Abadehellopsis longlinensis (Lin, Li & Sun, 1990) †
• Abadehella mengcunensis Wang, 2002 †
• Abadehella minima Wang in Zhao et al., 1981 †
• Abadehella regularis Lin, Li & Sun, 1990 † accepted as Abadehellopsis regularis (Lin, Li & Sun, 1990) †
• Abadehella regularisaeformis Lin, Li & Sun, 1990 † accepted as Abadehellopsis regularisaeformis (Lin, Li & Sun, 1990) †
• Abadehella tarazi Okimura & Ishii, 1975 †
• Abadehellopsis longlinensis (Lin, Li & Sun, 1990) †
• Abadehellopsis pauciseptata Vachard in Krainer, Vachard & Schaffhauser, 2019 †
• Abadehellopsis protea (Cushman & Waters, 1928) †
• Abadehellopsis regularis (Lin, Li & Sun, 1990) †
• Abadehellopsis regularisaeformis (Lin, Li & Sun, 1990) †
• Abas wittii (Grunow) R.Ross & P.A.Sims, 1980 †
• Abathomphalus asteroidalis Salaj, 1983 †
• Abathomphalus intermedius (Bolli, 1951) †
• Abathomphalus mayaroensis (Bolli, 1951) †
• Abathomphalus salajensis Solakius, 1983 †
• Abbottina tannerbankensis McCulloch, 1977 accepted as Nonion tannerbankensis (McCulloch, 1977)
• Abditodentrix asketocomptella Patterson, 1985 accepted as Abditodentrix pseudothalmanni (Boltovskoy & Guissani de Kahn, 1981)
• Abditodentrix pseudothalmanni (Boltovskoy & Guissani de Kahn, 1981)
• Abditodentrix rhomboidalis (Millett, 1899) accepted as Tortoplectella rhomboidalis (Millett, 1899)
• Abdullaevia tosbulakensis Suleymanov, 1965 †
• Abedinium dasypus (Cachon & Cachon-Enjumet) Loeblich Jr & Loeblich III, 1966
• Aberisphaera gambanica Wan, 1991 †
• Abollifer globosa Shiratori, Yokoyama & Ishida, 2014
• Abollifer prolabens Vørs, 1992
• Aboudaragina eponidelliformis Nakkady, 1955 † (uncertain > taxon inquirendum)
• Abrardia catalaunica Bilotte, 1974 †
• Abrardia mosae (Hofker, 1955) †
• Abratopdinium cardioforme S.Mao & B.A.R.Mohr †
• Abratopdinium kerguelense S.Mao & B.A.R.Mohr †
• Abriolina mediterranea Luperto, 1964 †
• Abrotanifolia barbata Stackhouse, 1809 accepted as Cystoseira barbata (Stackhouse) C.Agardh, 1820 accepted as Treptacantha barbata (Stackhouse) Orellana & Sansón, 2019
• Abrotanifolia discors (Linnaeus) Stackhouse, 1809 accepted as Cystoseira foeniculacea (Linnaeus) Greville, 1830
• Abrotanifolia foeniculacea (Linnaeus) Stackhouse, 1809 accepted as Cystoseira foeniculacea (Linnaeus) Greville, 1830
• Abrotanifolia loeflingii Stackhouse, 1809 accepted as Cystoseira foeniculacea (Linnaeus) Greville, 1830
• Abrotanifolia mucronata (Turner) Stackhouse, 1809 accepted as Gongolaria nodicaulis (Withering) Molinari & Guiry, 2020
• Abuhammadina saidi Abd-Elshafy & Ibrahim, 1990 †
• Abyssalia foliformis Gooday & Holzmann, 2020
• Abyssalia sphaerica Gooday & Holzmann, 2020
• Abyssamina incisa Schnitker & Tjalsma, 1980 †
• Abyssamina poagi Schnitker & Tjalsma, 1980 †
• Abyssamina quadrata Schnitker & Tjalsma, 1980 †
• Abyssotherma pacifica Brönnimann, Van Dover & Whittaker, 1989
• Acanthaulax aceras (Eisenack) Stover & Evitt (uncertain)
• Acanthaulax aculeata (Klement) Stover & Evitt (uncertain)
• Acanthaulax angulosa (Gitmez) Stover & Evitt (uncertain)
• Acanthaulax spinosissima (Deflandre) Sarjeant (uncertain)
• Acanthaulax venusta (Klement) Sarjeant (uncertain)
• Acanthoceras azoricum (Menegh.) Kütz., 1862 accepted as Ceramium azoricum Meneghini accepted as Ceramium echionotum var. azoricum (Meneghini) Piccone, 1884
• Acanthoceras echinophorum (Menegh.) Frauenf., 1855 accepted as Ceramium echionotum J.Agardh, 1844
• Acanthoceras echionotum (J.Agardh) Kütz., 1849 accepted as Ceramium echionotum J.Agardh, 1844
• Acanthoceras magdeburgense Honigm., 1910 accepted as Acanthoceras zachariasii (Brun) Simonsen, 1979
• Acanthoceras magdeburgense var. lata Honigm., 1909 accepted as Acanthoceras zachariasii (Brun) Simonsen, 1979
• Acanthoceras magdeburgense var. magdeburgense Honigmann, 1909 accepted as Acanthoceras magdeburgense Honigm., 1910 accepted as Acanthoceras zachariasii (Brun) Simonsen, 1979
• Acanthoceras oxyacanthum Kützing, 1849 (uncertain)
• Acanthoceras zachariasii (Brun) Simonsen, 1979
• Acanthochiasma fusiforme Haeckel, 1861
• Acanthochloris bacillifera Pascher, 1938
• Acanthochloris brevispinosa Pascher, 1930
• Acanthochloris brevispinosa var. brevispinosa Péterfi & Péterfi, 1966
• Acanthochloris hungarica Palik (uncertain)
• Acanthochloris ingens Margalef, 1956
• Acanthochloris scherffelii Pascher, 1938
• Acanthochloris verrucosa Ettl, 1977
• Acanthochrysis echinococcus Conrad (uncertain)
• Acanthochrysis echinocystus (Conrad) Conrad (uncertain)
• Acanthochrysis fusiformis Conrad (uncertain)
• Acanthocystis aculeata Hert. Less.
• Acanthocystis paliformis
• Acanthocystis pelagica
• Acanthocystis perpusilla
• Acanthocystis turfacea
• Acanthodesmia vinculata Mueller, 1858
• Acanthodesmia viniculata (Mueller) Mueller, 1858 accepted as Acanthodesmia vinculata Mueller, 1858
• Acanthodinium caryophyllum Kofoid, 1907
• Acanthodinium spinosum Kofoid, 1907 accepted as Cladopyxis spinosa (Kofoid) Schiller
• Acanthodiscus antarcticus Hajós, 1975 †
• Acanthodiscus clypeolus (Brun) Van Heurck, 1896 †
• Acanthodiscus convexus Hajós & Stradner in Hajós, 1975 †
• Acanthodiscus danicus J.Fenner, 1994 †
• Acanthodiscus immaculatus (Hanna) V.A.Nikolaev & J.A.Barron in Nikolaev et al., 2001 †
• Acanthodiscus ornatus Hajós & Stradner in Hajós, 1975 †
• Acanthodiscus paterus (J.A.Long, Fuge & J.Smith) V.A.Nikolajev & Fourt. †
• Acanthodiscus rugosus Pant., 1892 †
• Acanthodiscus schmidtii Jousé, 1951 †
• Acanthodiscus vulcaniformis Jousé, 1951 †
• Acanthogonyaulax spinifera (Murray & Whitting) H.W.Graham, 1942
• Acanthogonyaulax venusta (Klement) Sarjeant (uncertain)
• Acanthoica acanthifera Lohmann ex Lohmann, 1913
• Acanthoica aculeata Kamptner, 1941 accepted as Cyrtosphaera aculeata (Kamptner) A.Kleijne, 1992