Machaerina tenuissima Brady & Norman, 1889 – synonym of Paradoxostoma tenuissimum
Macrocythere simplex Sars, 1926 – synonym of Jonesia acuminata
Macromysis neglecta (G.O. Sars, 1869) – synonym of Praunus neglectus
Macromysis nigra Keeble and Gamble, 1904 – synonym of Praunus neglectus
Macropipus arcuatus Christiansen, 1969 – synonym of Liocarcinus arcuatus
Macropipus corrugatus Allen, 1967 – synonym of Liocarcinus corrugatus
Macropipus depurator Christiansen, 1969 – synonym of Liocarcinus depurator
Macropipus holsatus Christiansen, 1969 – synonym of Liocarcinus holsatus
Macropipus marmoreus Christiansen, 1969 – synonym of Liocarcinus marmoreus
Macropipus puber Holthuis & Gottlieb, 1958 – synonym of Necora puber
Macropipus pusillus Christiansen, 1969 – synonym of Liocarcinus pusillus
Macropipus vernalis (Risso, 1816) – synonym of Liocarcinus vernalis
Macropodia egyptia Allen, 1967 – synonym of Macropodia deflexa
Macropodia phalangium Leach, 1815a – synonym of Macropodia rostrata
Macropsis slabberi (Van Beneden, 1861) – synonym of Mesopodopsis slabberi
Macropus aracnides Risso, 1816 – synonym of Inachus phalangium
Maera batei Norman, 1868 – synonym of Maerella tenuimana
Maera brooki Scott, 1896 – synonym of Maera othonis
Maera tenuimana Stebbing, 1906 – synonym of Maerella tenuimana
Maia araneus Leach, 1814 – synonym of Hyas araneus
Maia bufo Bosc, 1801-1802 – synonym of Hyas araneus
Maja gigantea Baudouin, 1906 – synonym of Maja squinado
Maja squinado var. brachydactyla Balss, 1922 – synonym of Maja squinado
Marinogammarus finrnarchicus Sexton and Spooner, 1940 – synonym of Gammarus finmarchicus
Marinogammarus marinus Sexton and Spooner, 1940 – synonym of Gammarus marinus
Marinogammarus obtusatus Sexton and Spooner, 1940 – synonym of Gammarus obtusatus
Marinogammarus pirloti Sexton and Spooner, 1940 – synonym of Echinogammarus pirloti
Marinogammarus stoerensis Sexton and Spooner, 1940 – synonym of Echinogammarus stoerensis
Megamoera alderi Bate, 1862 – synonym of Abludomelita obtusata
Megamoera brevicaudata Bate, 1862 – synonym of Elasmopus rapax
Megamoera dentata Bate, 1862 – synonym of Melita dentata
Megamoera longimana Bate, 1862 – synonym of Maera othonis
Megamoera othonis Bate, 1862 – synonym of Maera othonis
Meganyctiphanes calmani Colosi, 1918 – synonym of Meganyctiphanes norvegica
Melita fucicola Della Valle, 1893 – synonym of Gammarella fucicola
Melita obtusata Bate, 1862 – synonym of Abludomelita obtusata
Melita pellucida Sars, 1882 – synonym of Allomelita pellucida
Melita proxima Bate, 1862 – synonym of Abludomelita obtusata
Melita reidi Hamond, 1965 – synonym of Abludomelita obtusata
Melphidippa macra Norman, 1889 – synonym of Melphidippella macra
Melphidippella macera Sars, 1894 – synonym of Melphidippella macra
Mesidotea entomon Richardson, 1905 – synonym of Saduria entomon
Metaphoxus fultoni Stebbing, 1906 – synonym of Parametaphoxus fultoni
Metopa abscisa Norman, 1900 – synonym of Metopa latimana
Metopa nasuta Boeck, 1871 – synonym of Hardametopa nasuta
Metopa rubrovittata Sars, 1882 – synonym of Stenula rubrovittata
Metopella nasuta Stebbing, 1906 – synonym of Hardametopa nasuta
Microcheles armata Kröyer, 1846 – synonym of Iphimedia obesa
Microdeuteropus anomalus Norman, 1869 – synonym of Microdeutopus anomalus
Microdeuteropus gryllotalpa Norman, 1886 – synonym of Microdeutopus gryllotalpa
Microdeuteropus minax Norman, 1886 – synonym of Microdeutopus gryllotalpa
Microdeuteropus versiculatus Norman, 1869 – synonym of Microdeutopus versiculatus
Microdeutopus grandimanus Bate, 1862 – synonym of Microdeutopus gryllotalpa
Microdeutopus propinquus Sars, 1894 – synonym of Microdeutopus anomalus
Microdeutopus websteri Bate and Westwood, 1862 – synonym of Lembos websteri
Monoculodes falcatus Sars, 1892 – synonym of Monoculodes subnudus
Monoculodes longimanus Bate and Westwood, 1868 – synonym of Perioculodes longimanus
Monoculodes tenuirostratus Boeck, 1871 – synonym of Monoculodes packardi
Montagua alderi Bate, 1857 – synonym of Metopa alderi
Montagua bruzelii Goes, 1866 – synonym of Metopa bruzelii
Montagua clypeata Bate, 1862 – synonym of Metopa latimana
Montagua monoculoides Bate, 1856 – synonym of Stenothoe monoculoides
Montagua norvegica Bate and Westwood, 1868 – synonym of Metopa alderi
Montagua phyllonyx Bate, 1862 – synonym of Arrhis phyllonyx
Munna (Neomunna) kroeyeri Goodsir, 1842 – synonym of Munna kroyeri
Munna kroeyeri Sars, 1897 – synonym of Munna kroyeri
Munna whiteana Bate & Westwood, 1868 – synonym of Munna kroyeri
Mysidopsis hispida Norman, 1869 – synonym of Leptomysis gracilis
Mysis chamoeleon (Müller, 1776) – synonym of Praunus flexuosus
Mysis didelphys Norman, 1863 – synonym of Mysidopsis didelphys
Mysis erythrophthalma Goës, 1864 – synonym of Erythrops erythrophthalma
Mysis flexuosa (Müller, 1776) – synonym of Praunus flexuosus
Mysis gracilis G.O. Sars, 1864 – synonym of Leptomysis gracilis
Mysis griffithsiae Bell, 1853 – synonym of Siriella armata
Mysis kervillei G.O. Sars, 1885 – synonym of Schistomysis kervillei
Mysis lamornae Couch, 1856 – synonym of Hemimysis lamornae
Mysis leachii Thompson, 1828 – synonym of Praunus flexuosus
Mysis lingvura G.O. Sars, 1866 – synonym of Leptomysis lingvura
Mysis neglecta G.O. Sars, 1869 – synonym of Praunus neglectus
Mysis ornata G.O. Sars, 1864 – synonym of Schistomysis ornata
Mysis parkeri Step: 1896 – synonym of Schistomysis parkeri
Mysis productus Gosse, 1853 – synonym of Siriella armata
Mysis rostrata Guérin-Meneville, 1844 – synonym of Siriella armata
Mysis sancta Van Beneden, 1861 – synonym of Gastrosaccus sanctus
Mysis scoticus J.V. Thompson, 1828 – synonym of Neomysis integer
Mysis serraticauda Walker, 1901 – synonym of Mysidopsis angusta
Mysis slabberi (Van Beneden, 1861) – synonym of Mesopodopsis slabberi
Mysis spinifera Goës, 1864 – synonym of Gastrosaccus spinifer
Mysis spinulosus Leach, 1815 – synonym of Praunus flexuosus
Mysis spiritus Norman, 1860 – synonym of Schistomysis spiritus
Mysis vulgaris J.V. Thompson, 1828 – synonym of Neomysis integer