A new species of Chaetiliidae (Crustacea: Isopoda: Valvifera) from the Río de la Plata estuary, Argentina-Uruguay, and reconsideration of Macrochiridothea and Chiriscus Author Poore, Gary C. B. text Zootaxa 2009 2119 51 65 journal article 10.5281/zenodo.188066 ab6365d5-73f9-4961-b482-3020feb144a3 1175-5326 188066 Chiriscus Richardson, 1911 Chiriscus Richardson, 1911 : 169 –170. — Sheppard, 1957 : 168 –169. — Jones & Fenwick, 1978 : 619 . Macrochiridothea (part). — Poore, 1984 : 71 –72. Type species. Chiriscus australis Richardson, 1911 , by original designation. Diagnosis. Body smooth. Head with or without lateral incisions (very short if present). Eyes present. Pereonites with dorsal coxal plates visible dorsally on 5–7 only. Pereonite 7 tergite about 3 times as wide as long; pleonites 1–3 free. Coxa 7 significantly smaller than coxa 6. Antenna 1 peduncle article 2 without anterodistal prolongation; article 3, 1.2 times as long as article 2. Antenna 2 peduncle article 4 greatest width twice length of anterior margin, with a broad posterodistal lobe extending halfway alongside article 5. Mandibles without molar; incisor with 4 acute teeth on left, of 5 uneven teeth on right; lacinia mobilis with concave, denticulate margin on left, of 4 blunt teeth on right; spine row of denticulate spines. Maxillipedal palp of 4 articles, articles 4 and 5 free. Pereopod 1 with grossly widened propodus, about as wide as long, much wider than propodus of pereopods 2 and 3. Pereopods 2 and 3 basis anterior margin with setae only on proximal half; carpus posterodistal corner grossly produced as a lobe (about half as long as and dominating propodus), bearing long distal setae; dactyli absent. Pereopod 4 dactylus vestigial, dome-like, with a minute terminal seta. Pereopod 5 dactylus short, cylindrical, with a long terminal seta; pereopod 6, 1.3 times as long as pereopod 5. Pereopod 7 dactylus minute, dome-like, with a short apical seta. Pleopod 2, appendix masculina straight, with lobulate apex. Uropod endopod half length of exopod. Oostegites 1 broad overlapping plates, longer than broad; oostegites 2–4 almost circular, overlapping; oostegites 5 linguiform, reaching midline. Remarks. In reviving Chiriscus we recognise distinctive character states in the two species we now assign to this genus that differentiate them from Macrochiridothea s.s. The absence of pereopodal dactyli on pereopods 2–5 and 7 that inspired Richardson to erect her genus proved to be true only for pereopods 2 and 3 ( Poore, 1984 ) but the shape of antenna 2 (wide article 4 and with a broad posterodistal lobe) and long pereonite 7 are distinctive. All species of Macrochiridothea possess dactyli on pereopods 2 and 3. Males of only one species are known but its appendix masculina is of a characteristic form different from those of other chaetiliid genera. Poore (1984) transferred Chiriscus australis to the larger genus Macrochiridothea on the basis of numerous shared features. At the time, the differences were treated as unique features. He overlooked the similar species, Macrochiridothea giambiagiae Bastida & Torti, 1972 now placed with C. australis in the revived Chiriscus .