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.
Rhabdomesus: Richardson, 1908: 81.
Bactromesus: Wolff, 1962: 83.
Type species: Ischnosoma bispinosum G. O. Sars, 1868 (by monotypy).
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. justi Merrin & Poore, 2003; 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.
Diagnosis. Pereonites 5–7, pleonite 1 and pleotelson freely articulating. Antennula of at least 6 articles. Maxillipedal palp width equal to or wider than basal endite. Pereopod I carpus with weakly expanded palm. Uropods uniramous, biarticulate.
Remarks. 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 and pleotelson fused together in genus Haplomesus; see Kavanagh & Wilson 2007). Uniarticulate uropods have been described by Merrin & Poore ( 2003) for Ischnomesus justi. As this species displays all the other diagnostic characters that characterise Ischnomesus, biarticulate uropods may not be a diagnostic character for the genus. However, as all remaining 35 species and I. harrietae sp. nov. possess uropods with two articles, we have retained it as a generic character until further information on the occurrence of uniarticulate uropods becomes available through the description of additional species.