Type species. Ultimachelium tac sp. nov.
Etymology. The generic name is contrived.
Diagnostic description. Body shape slender. Head lateral cephalic lobe large, broadly or narrowly rounded. Antenna 1 peduncular article 1 without posterodistal spine; accessory flagellum article 1 long; flagellum with callynophore in female. Mandible with accessory setal row absent; left lamina dentata absent; palp attached distally; palp article 2 broad, article 3 with subparallel margins or tapered distally. Maxilla 1 outer plate setal-teeth smooth, at least one inner row setal-tooth absent; palp absent. Maxilliped inner plate absent; outer plate medium size; palp 4-articulate, article 4 large. Gnathopod 1 ischium slightly smaller than, subequal to or larger than propodus; corner of palm without robust seta. Gnathopod 2 palm obtuse. Epimeron 3 posteroventral corner projecting, narrowly rounded. Uropod 3 biramous.
Species composition. Ultimachelium contains four species: U. barnardi (Alonso de Pina, 1993); U. nichollsi (Lowry, 1984); U. schellenbergi (Lowry, 1984); U. tac sp. nov.
Remarks. Species in Ultimachelium were previously assigned to Pachychelium. In our view the old concept of Pachychelium is a complex of three genera: Pachychelium in the North Atlantic and North Pacific with one species in the deep-water tropics of north-eastern Australia; Smaraldia, a monotypic genus currently confined to southern temperate Australia; and Ultimachelium in southern South America and the Antarctic.