Ischnosoma G.O. Sars, 1868: 84.
Ischnomesus Richardson, 1908: 81;— Hansen, 1916: 56;— Gurjanova, 1932: 40 l;— Wolff, 1956: 88;— Menzies, 1962: 111;— Wolff, 1962: 73;— Menzies & George, 1972: 971;— Kussakin, 1988: 419;— Merrin & Poore, 2003: 290;— Kavanagh et al., 2015: 203.
Rhabdomesus Richardson, 1908: 81.
Bactromesus Wolff, 1962: 83.
Type species: Ischnosoma bispinosum G.O. Sars, 1868 (by monotypy).
Diagnosis. See Kavanagh et al. (2015).
Species included: Ischnomesus anacanthus Wolff, 1962; I. andriashevi Birstein, 1960; I. antarcticus Schultz, 1979; I. armatus Hansen, 1916; I. bacilloides ( Beddard, 1886); I. bacillopsis ( Barnard, 1920); I. bacillus ( Beddard, 1886); I. bidens Menzies, 1962; I. birsteini Wolff, 1962; I. bispinosus ( G.O. Sars, 1868); I. bruuni Wolff, 1956; I. calcificus Menzies & George, 1972; I. caribbicus Menzies, 1962; I. carolinae Chardy, 1974; I. chardyi Kussakin, 1988; I. decemspinosus Menzies, 1962; I. elegans Menzies, 1962; I. elongatus Birstein, 1963; I. fragilis Birstein, 1971; I. glabra Kensley, 1984; I. gracilis ( Birstein, 1960); I. harrietae Kavanagh, Frutos & Sorbe, 2015; I. justi Merrin & Poore, 2003; I. kussakini nom. nov. (Kussakin, 1982); I. latimanus Birstein, 1971; I. magnificus Menzies, 1962; I. multispinis Menzies, 1962; I. norvegicus Svavarsson, 1984; I. paucispinis Menzies, 1962; I. planus Wolff, 1962; I. profundus Hansen, 1916; I. roseus Wolff, 1962; I. simplex Menzies & George, 1972; I. simplissimus Menzies, 1962; I. spaercki Wolff, 1956; I. tasmanensis Merrin & Poore, 2003; I. vinogradovi Birstein, 1963 and I. wolffi Menzies, 1962.
Remarks. According to Kavanagh et al. (2015), the freely articulating posterior pereonites, pleonite 1 and pleotelson observed in Ischnomesus is a condition presumed to be plesiomorphic, with other genera showing various degrees of fusion of the pereonites (e.g. pereonites 5–7, pleonite and pleotelson fused together in genus Haplomesus; Kavanagh & Wilson 2007). Uniarticulate uropods have been described by Merrin & Poore (2003) for Ischnomesus justi, so the biarticulate uropods may not be a diagnostic character for the genus, although all other species, described with this structure, have biarticulate uropods.