(Figs 44, 45)
Pachychelium antarcticum. ― Nicholls, 1938: 14, fig. 3. ― Bellan-Santini, 1972: 215, pl. 28. ― Lowry & Bullock, 1976: 101 (in part). ― Lowry, 1982: 320.
Pachychelium nichollsi Lowry, 1984: 103. ― Barnard & Karaman, 1991: 510. ― De Broyer & Jazdzewski, 1993: 72.
Ultimachelium nichollsi. ― De Broyer et al., 2007: 158.
Material examined. One?female, 12 mm, USNM 1156710, off South Georgia, Southern Ocean, 54°51'S 35°03'W, 168–173 m, trawl, 10 December 1986, Professor Siedlecki, USARP cruise 601, stn 74. –1?male, 10.0 mm, USNM 1157037, unknown locality, stn AH 4-30. –1?male, 11.5 mm, USNM 1157039, unknown locality, stn AH 4-40.
Diagnostic description. Head lateral cephalic lobe large, narrowly rounded; ventral margin subsigmoidal. Antenna 1 peduncular article 1 without posterodistal spine; accessory flagellum 2-articulate, article 1 long; flagellum with strong callynophore in specimens of unknown sex. Mandible lacinia mobilis absent; lamina dentata absent; accessory setal row absent; molar absent; palp attached distally; palp article 2 broad, as long as article 3; article 3 tapered distally. Maxilla 1 outer plate with 6 setal-teeth in 3/3 arrangement, setal-teeth smooth; palp absent. Maxilliped inner plate absent; outer plate medium size; palp 4-articulate, article 4 large. Gnathopod 1 subchelate; ischium smaller than propodus; merus with anterior margin; carpus extremely compressed; propodus length about 1.9 × breadth, posterior margin extremely reduced, with produced corner, corner of palm without robust seta; dactylus reaching to corner of palm. Gnathopod 2 palm minute, obtuse. Pereonite 4 without dorsal spines. Pereonite 5 without dorsal spines. Pereopod 5 merus without posteroproximal shoulder. Pereopod 6 basis with posteroventral lobe weakly produced; merus longer than broad, without posteroproximal shoulder. Pereopod 7 basis subrectangular, posterodistal corner subquadrate , posterior margin weakly dentate; merus, length of anterior margin 2.6 × proximal breadth. Epimeron 3 posterior margin without projections, posteroventral corner subacute. Uropod 3 biramous, inner ramus just reaching base of article 2 or extending beyond article 1 of outer ramus. Telson slightly shorter than broad, entire.
Remarks. Ultimachelium barnardi and U. nichollsi both have six setal-teeth on the outer plate of maxilla 1 in a 3/3 arrangement. They differ in the shape of mandibular palp article 3 (subparallel sides in U. barnardi, tapering distally in U. nichollsi); and gnathopod 1 ischium (swollen posteroproximally in U. nichollsi, not swollen in U. barnardi).
Distribution. South Georgia, Southern Ocean and Adelie Coast, Antarctica; 46–732 m depth.