(Figures 19, 20)

Halosydna patagonica Kinberg, 1856: 385; 1858: 17, plate 5, figure 23; Hartman, 1949: 16–17, plate 2, figures 15–18.

Holotype (SMNH 408), York Bay, Magellan Sound, Chile, 53 ◦ 34 ′ S, 072 ◦ 20 ′ W, Expedition Eugenie 1851-53.

Holotype in poor condition, in two fragments, lacking some median segments. Anterior fragment with 15 segments, posterior fragment with 11 segments (0.65 + 0.7 cm length, 0.55 cm width), body subrectangular in cross-section, pale-yellow.

Prostomium bilobed (Figure 19A), without cephalic peaks; facial tubercle absent; two pairs of eyes, anterior pair on widest part of prostomium, posterior pair near posterior margin; median antenna with ceratophore, inserted frontally, style without papillae expanded subdistally, tip filiform; lateral antennae ceratophores inserted terminally as prolongations of prostomium lobes, at same level as median antenna ceratophore, styles subdistally tapering to filiform tips, surfaces smooth. Palps robust, surfaces smooth. Pharynx dissected and missing.

Tentacular segment not visible dorsally; tentaculophores with slender chaetae; tentacular cirri similar to antennae. Segment 2 projects slightly over prostomium.

Body with 15 pairs of elytrophores. Anterior fragment with eight pairs of elytrophores inserted on segments 2, 4, 5, 7, 9, 11, 13, 15. Posterior fragment with five pairs of elytrophores inserted on segments 27, 28, 30, 31, 33. Three posteriormost segments with dorsal cirri. First pair of elytra circular (Figure 19B), marginal papillae short, scarce; surfaces with macrotubercles, conical, sclerotized, scattered, smaller along margins, longer and curved over elytrophore scar (Figure 19B,C). Elytra of third pair with marginal papillae short and microtubercles scarce on surfaces. Microtubercles distally blunt, surfaces rough, with areola basally thicker and blunt on elytrophore scar. Elytra of median and posterior segments oval, with marginal papillae (Figure 19D–F), microtubercles thick, shorter.

Notopodia shorter than neuropodia. Neuropodia distally truncate (Figure 20A,B), protruding acicular tips, prechaetal lobe with small rounded lobes near acicular tips. Elytrophores wider than dorsal tubercles. Dorsal cirri with surfaces smooth, similar to antennae. Cirrophores basally expanded. Ventral cirri taper to filiform tips. Nephridial papillae elongated in median and posterior segments. Anus dorsal. Pygidium with two anal cirri similar to dorsal cirri. Pharynx not everted.

Notochaetae shorter than neurochaetae, with rows of spines; the smaller curved (Figure 20F), tips blunt; remaining ones tapering to capillary tips (Figure 20G).

Neurochaetae with rows of spines on upper region (Figure 20C), tips bidentate, main tooth curved, subdistal tooth shorter (Figure 20D,E).

The holotype was originally described as having 36 segments and 18 pairs of elytra. It was labelled as syntype, but the description was based in a single specimen, which must be considered as the holotype. Halosydna patagonica has not been recorded in the Grand Caribbean or tropical eastern Pacific.

York Bay, Magellan Sound, Chile, 53 ◦ 34 ′ S, 072 ◦ 20 ′ W.

Same as type locality.