The arcturid genus Arcturopsis Koehler, 1911 is rediagnosed and a key to its three species given. The type species, Arcturopsis giardi herein designated, is redescribed from new material. Arcturopsis is diagnosed as follows: body geniculate; margins of head and pereonite 1 separated by lateral incision; antenna 2 slender, with flagellum having a row of spines medially and ending in a claw; pereopod 1 with dactylus and unguis; pereopods 2 to 4 cylindrical and slender with paired closely spaced long setae, flexed between the carpus and propodus, dactylus absent; pereopods 5 to 7 with secondary unguis fused, smaller than the primary unguis; pleotelson with small lateral wing; oostegite 4 with posterior lobe; male more elongate than female, especially pereonite 4; pereonite 3 possessing a midventral appendage with complex apex; male pleopod 1 exopod laterally notched, with three long proximal setae on posterior face; male pleopod 2 with appendix masculina elongate, exceeding endopod, with two apical flagellate appendices. Post-marsupial developmental stages of Arcturopsis giardi are described. The species is distinguished from the A. rudis and A. senegalensis by having a female pereonite 4 without dorsal paired horns, longer than wide, and about seven times as long as pereonite 3, an apically blunt pleotelson, an antenna 2 flagellum of two articles plus a claw, and a male with a tridentate midventral appendage on pereonite 3.
Crustacea, Isopoda, Valvifera, Arcturidae, Arcturopsis, North Atlantic Ocean, diagnosis, redescription.