Figures 1a, b; 4a
Psolus disciformis Théel, 1886: 85, pl. 9 fig. 6.
Theelia disciformis.— Ludwig, 1892: 350.— Perrier, 1902: 494.
Theelia (?Psolidium) disciformis.— Ludwig, 1894: 136.— Ludwig, 1898: 52.
Psolidium disciformis.— Deichmann, 1947: 337.— Pawson, 1969: 38, map 3.
Material examined. Syntypes (2): Chile, Challenger stn 311, 52°45’30"S 73°46’W, 448 m, BM(NH) [18]86.10.2.86.
Diagnosis. Psolidium species up to 18 mm long (25 mm in Théel, 1886); dorsal and lateral scales conspicuous, up to 1 mm wide; dorsal and lateral tube feet numerous, conspicuous, up to 2 penetrate each scale.
Sole: outer peripheral single series of smaller tube feet; inner peripheral single series of larger tube feet; zig-zag series of mid-ventral (sole) radial tube feet.
Sole ossicles: numerous irregularly oval to round, disc-like smooth plates, variably with few small perforations, up to 220 μ m long; rare shallow cupped crosses and more numerous 4-holed regular shallow cups with blunt marginal spines, cups 150 μ m long.
Colour (live). White (P. Lambert, pers. comm.).
Distribution. SE Pacific Ocean, Chile, Comau Fiord to Strait of Magellan (42° to 56°S), on rock; 8–448 m (P. Lambert, pers. comm.).
Remarks. Théel (1886) noted the presence of dorsal and lateral tube feet in his new species, but Ludwig (1886) erected the genus Psolidium for Psolidae with this diagnostic character in the same year. Deichmann (1947) assigned Psolus disciformis Théel, 1886, to Psolidium Ludwig, 1886. The form of the ossicles in the sole is diagnostically distinctive, and ossicles from dorsum and tentacles were not examined. Recent intensive field work on the Chilean coast under the auspice of the San Ignacio del Huinay Foundation has discovered an abundance of P. disciformis on the rocks in the coastal shallow from 8 to 27 m (P. Lambert, pers. comm.).