Mediaster sp. (Fig. 1, h-k; Pl. 2, e, f).

Material Examined:

One specimen, Sta. 1818.

Size:

R/r = 25/12 mm.

Description:

Disc flat, arms 5, short, tapering to sharp tip protected by small plate.

Abactinal surface paved by small, regularly arranged (especially along midline of arms) round or oval, gently lobed plates (fig. h); each plate bears a short thick stem or trunk crowned (centrally on disc) by 5–11 (generally 8) small, thick almost triangular-shaped granules or short spines, generally one (sometimes as many as 4) similar granules centrally. Along midline of arms paxillae similar although smaller with generally only 8 or 9 peripheral granules and either none, or as many as 2, centrally; along arm edges paxillae small with often only a single row of 4 or 5 granules.

Papulae (fig. h) conspicuous along midline of arms, less conspicuous on disc centre, absent from narrow area interradially; generally 6 papulae surround each plate.

Madreporite interradial, midway between centre and edge of disc, small, deeply dissected, almost square. Paxillae immediately adjacent enlarged, with up to 16 spines.

Anus central on disc, small, inconspicuous, guarded by 5 or 6 slender spines.

Marginal plates conspicuous bordering arms and discs; 15–17 plates from interradial angle to arm tip. Superomarginals rectangular, gently tumid, with uniform covering of small, round, spinulose granules which are easily rubbed off leaving fine pits. No enlarged granules or spines.

Inferomarginals (Fig. j) corresponding to superomarginals, slightly larger, forming distinct border to actinal surface; plates with covering of granules similar to those of superomarginals.

Actinal interradial areas well developed, triangular, with plates (figs, i, j) extending for at least half length of each arm, to 7th or 8th marginal plate. Plates either rectangular or irregular in shape; adjacent to adambulcral plates regularly arranged, elsewhere irregular, each plate with, centrally and near adambulcrals, 9–11 small, squat, triangular-headed granules; near inferomarginals and along arms granules fewer, 4–8, smaller. No enlarged spines or granules; central granules on plates not enlarged. On actinal plates immediately adjacent to oral plates in all angles, single small pit present.

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Adambulacral plates (fig. i) very regular with fringe of 6 or 7 (exceptionally 8) furrow spines; most proximal spine of plate smaller, set slightly in from plate edge; near oral plates it may be less than half length of first true furrow spine. Immediately adjacent to furrow spines row of 4 or occasionally 5 thicker, shorter, almost club-shaped subambulacral spines or granules and behind these from 5–8 even shorter spines arranged in one or 2 more or less regular longitudinal rows.

No pedicellariae. Oral furrow spines 11, rather flattened, of similar size, except for most distal spine adjacent to first adambulacral which is distinctly shorter; suboral spines 14 with fairly regular row of 7 or 8 spines bordering suture, rest of spines irregularly arranged.

Tube feet biserially arranged with small sucking discs.

Anatomy

A small part of the abactinal surface (fig. k) was reflected. Abactinal plates, in papular area, connected by slender rods, midline of arms with conspicuously regular arrangement of plates; rudimentary superambulacral plates present but inconspicuous, only clearly visible in midpoint of arm; interbrachial septa membranous, gonads small.

Remarks:

The present specimen can be distinguished from the single New Zealand representative, M. sladeni (Benham), by the difference in body form, the more numerous adambulacral furrow spines and the apparently fewer oral furrow spines. It is not dissimilar to M. ornatus Fisher from Hawaiian waters, but it differs in details of the abactinal and actinal armature and in lacking pedicellariae. This specimen resembles M. arcuatus (Sladen) from Japan but differs in lacking pedicellariae (this may be due to the small size of the present specimen) and there are also small differences in the armature of the abactinal and actinal plates.

Colour:

Abactinal surface pale pink marginally, white to pale blue-grey centrally; actinal surface white, faintest pink marginally and ambulacral grooves and tube feet pale yellow.

Geographical Distribution

Known only from "Eltanin" Sta. 1818, off the West Coast of the South Island of New Zealand.

Depth Range:

908–915 m.

Locality:

"Eltanin" Sta. 1818, 40° 15′–40° 17′ S.; 168° 16′–168° 18′ E.; 908–915 m.

Location of Specimen

United States National Museum.