Type species. Porcellidium viride (Philippi, 1840).

Diagnosis [an alternative (apomorphic) character state is known for all characters marked with an asterisk]. Spermatophore elongate* (sausage shaped), female receives only one spermatophore during her life span, usually deposited on ventral surface of the genital double-somite at time of metamorphosis from stage V copepodid to adult, attachment to female ephemeral (shed before egg laying starts); bodY oval*, dorsoventrallY compressed*; male cephalosome truncated anteriorly*, female not truncated anteriorlY*; animals do not conglobate*; dorsal organs absent*, no massive honeYcomb-like cuticle; hYaline border surrounds cephalosome*, marginal glands open dorsal to hYaline border*; female genital double-somite broad*, divided into anterior and posterior lobes*, rounded posteriorlY*, lateral striations absent*; male genital somite never fused to metasome segment 4 and P5 baseoendopod*; female caudal ramus rectangular*, setae T1 to T4 always present* (T1 may be recessed), never pinnately clavate or evenlY spaced*; no ridge-plates on labrum*; six setae on maxillule endopod*; maxilliped coxae touch in midline*, basis with fimbriate process*; limbs P2, P3, P4 with three external spinous setae on exopod segment 3*; P5 shorter than genital double-somite*, ventral expansion absent*; male antennule segment 3 setae δ and δ' alwaYs present*, anterior comb absent*, blade or knob-like ventral process may be present, segment 4 with three denticles*, brush-pad never present*; male P5 trapezoid with six setae*.

Species composition. Porcellidium viride (Philippi, 1840); P. fimbriatum Claus, 1863; P. roscoffensis (Bocquet, 1948) comb. nov. (northeast Atlantic coast of Europe and Mediterranean Sea); P. rubrum Pallares, 1966 (Argentina); P. erythrum Hicks, 1971 (Aotearoa, New Zealand); P. hartmannorum Tiemann, 1978; P. algoense Hicks, 1982 (East and West coast of South Africa); P. hormosirii Harris & Robertson, 1994; P. ocellum Harris & Robertson, 1994; P. pulchrum Harris & Robertson, 1994; P. erythrogastrum Harris & Robertson, 1994; P. naviculum Harris & Robertson, 1994; P. phyllosporum Harris & Robertson, 1994; P. londonarum Harris, 1994 (name correction for P. londonii by Wells, 2007); P. rastellum sp. nov. (East Coast of Australia); P. ofunatense Harris & Iwasaki, 1996; P. kiiroum Harris & Iwasaki, 1996; P. akashimum Harris & Iwasaki, 1996; P. wandoensis Kim & Kim, 1997; P. brevicavum Kim & Kim, 1997 (Japan and Korea).