(Figures 17 A–E)
Diagnosis. Carinal series clearly distinguished, strongly convex, each plate with a periphery of ~ 20 coarse, sphaerical granules with 8–12 granules covering the plate surface (20–30 total on each plate surface). Superomarginal plates elongate with a strongly convex, round area free from granules, occupying 50 – 50 % of each supeormarginal plate surface. Furrow spines 2–3. Body weakly stellate (R/r= 1.74).
Etymology. The species epithet “ calvus ” refers to the Latin for “bald” which refers to the prominent bald spots on the superomarginal plate surface.
Distribution. Burdwood Bank in the Scotia Sea from 339– 357 m.
Material examined. HOLOTYPE. USNM 1149357. Burdwood Bank. 54 ° 43 ’S; 56 ° 37 ’W to 54 ° 42 ’S; 56 ° 36 ’W. 339– 357 m. Coll. R/V Eltanin, University of Southern California, 14 March 1966.(1 dry spec., R= 4, r= 2.3).
Description. Body form stellate-pentagonal (R/r= 1.74). Interadial arcs concave. Arms triangular (Fig. 17 A).
Abactinal plates rounded to irregular in outline, larger proximally becoming smaller and more irregular closer to contact with superomarginal plate series. Plates flattened with fasciolar grooves absent. Carinal plate series relatively nondescript relative to other abactinal plates, and are covered with granules proximally, except at distalmost regions on arms. At approximately nine to 10 carinals away from the terminus, these plates become bare and strongly convex, irregularly round in outline. All other abactinal plates covered by granules, 15–45 in number, rounded to weakly pointed (Fig. 17 B). Granules crowded but evenly spaced and are largest centrally with smaller granules forming periphery. Boundary between central and peripheral granules weakly defined. Papulae present in radial regions where they surround abactinal plates, six to a plate. Papulae absent interradially and centrally on disk.
Marginal plates 28 in each interradius (armtip to armtip), elongate in shape (Fig. 17 A). Superomarginal and inferomarginal plates with 1: 1 correspondance, each series forming distinct abactinal or actinal facing border (Fig. 17 D). Superomarginal plate surface, each with large bare, oblong smooth, swollen convexity adjacent to disk (Fig. 17 C, E). This convexity occupies up to 50–90 % of the superomarginal plate surface and is surrounded by 20–40 round, spherical to pointed granules similar in appearance to those on the disk (Fig. 17 D). These convexities are increasingly absent of granules as they are closer to the armtips (Fig. 17 C). Two distalmost superomarginals are in direct contact abutting over the midline on one arm but not on the others (Fig. 17 C). Granules, round, spherical occurring on surface of superomarginal adjacent to inferomarginal series becoming confluent with granules on lateral side of inferomarginal plates. These granules similar to those on disk, widely and evenly spaced but slightly larger. Inferomarginals with similar but smaller bare convex bare spot on plate surface facing ventrally (Fig. 17 E). This bare spot relatively small interradially becoming larger and occupying more of the inferomarginal plate surface distally. Greater amount of inferomarginal surface covered by granules (20–30 in number).
Actinal surface with quadrate-shaped actinal plates forming approximately six chevrons (Fig. 17 E). Largest and most complete chevron adjacent to adambulacral plate series extending to near arm base. Actinal plates quadrate but becoming more irregular adjacent to inferomarginal plate series. Each plate covered with spherical granules, 9–11 in number, evenly and widely spaced on each plate but becoming more densely packed adjacent to inferomarginal series. Furrow spines, 2–3 per adambulacral plate, slender, but quadrate in cross-section. Third spine is typically the smallest and directed proximally relative to the mouth. Subambulacral spines, two, blunt, conical, separated from the furrow spines by a discrete bare space. Behind the subambulacral spines are two to four conical to blunt granules which are intermediate with the rounded granules present on the actinal plate region. Oral plates with spines, four to five, triangular to quadrate in cross-section. One spine, each triangular in cross-section, from each plate projects into the mouth (one pair per interradius). Oral plate surface bare, covered by tissue bearing four to five spines, each with triangular-rounded spines in cross-section.
No pedicellariae were observed.
Comparisons with other taxa. Pillsburiaster calvus n. sp. is the first known Pillsburiaster species from the Antarctic Peninsular region and the first from the South Atlantic. This species differs from other known species of subantarctic Pillsburiaster in that it has three furrow spines rather then 5 or 6 as are present in the New Zealand Pillsburiaster aoteanus and P. indutilus and the five to seven furrow spines present in P. geographicus from the tropical Atlantic.