Table 1, Figures 3 a–f.
Synonymies. See Pawson (1969).
Material examined. Chile, Tierra del Fuego, Isla Dawson, 53 ° 58 'S 70 ° 34 'W, Gallardo, A. & Castillo, J., 26 Sep 1965, USNM E 31749 (1); Strait of Magellan, 1500–1666 m, USNS Eltanin, 1966, USNM E 33617 (2); South Georgia, 659–686 m, USNS Eltanin, 1966, USNM E 33616 (4); Australia, Macquarie I., off Lusitania Bay, 54 ° 43 ' 30 ''S 158 ° 53 '06''E, 100–105 m, ANARE, MV Nella Dan, Cochrane, T., 6 Dec 1986, NMV F 76117 (1).
Description. Psolus species up to 51 mm long, 35 mm wide, 12 mm high (damaged specimen, USNM E 31749, preserved); oval to round in form with low profile, slightly raised oral and anal cones; clear demarcation between the body wall scales and oral and anal openings, 5 large oral valves and 5 smaller anal valves, sometimes composite, with little to no encroaching by nearby body wall scales; tentacles 10, 8 large, 2 small ventrally; large, irregular dorso-lateral scales up to 10 mm long with finely granuliform ‘beaded’ surface, 2 to 3 rows of significantly smaller ventro-lateral margin scales, 1–3 mm long; sole with inner paired to zig-zag rows of larger tube feet and scattered smaller peripheral tube feet, bare mid-ventrally though tube feet extend variably at oral and anal ends onto the mid-ventral line.
Dorsal and lateral ossicles consist of multi-layered plates up to 1.2 mm long and perforated plates up to 0.2 mm long, variably knobbed with digitiform margins. Sole ossicles consist of irregular, heavily knobbed, perforated plates with raised margins either bluntly spinous, forked or knobbed, irregular in shape and up to 200 µ m long by 160 µ m wide with rounded perforations up to 40 µ m in diameter. Tentacle ossicles irregular perforated plates, variably smooth to knobbed, thick to thin, often with bluntly spinous edges; largest tentacle ossicles up to 208 µ m long by 184 µ m wide.
Colour (preserved). White.
Distribution. South America (Magellanic region), South Georgia, Australia (Macquarie I.); 100–1666 m.
Remarks. Pawson (1968) extended the geographical distribution of Psolus antarcticus to Australia. With no type specimen our description is based on a damaged specimen from what we understand to be the type locality (Tierra del Fuego). Specimens of P. antarcticus from Macquarie Island were also examined and compared with those from the type locality, Strait of Magellan, and South Georgia. Some Macquarie Island specimens were found to be morphologically conspecific with those from the type locality, with uniformly white scales, no encroaching of body wall scales on the oral valves, and knobbed plates rather than bowls in the sole ossicles. These have been treated as the same species for this key, as we await confirmation from molecular data. Others were designated as P. parantarcticus sp. nov. and are described below. P. a n t a rc t i c u s from both the type locality and Macquarie Island is similar to P. neozelanicus Mortensen, 1925 but is distinguished in the literature by having sole ossicles with marginal knobs and fewer perforations. P. neozelanicus is a small (up to 10 mm), rare species which has been found only off the northern tip of New Zealand.