Abyssaster diadematus (Sladen 1883)

“HYPHALASTER DIADEMATUS, n. sp.

Marginal contour stellato-pentagonal. Rays five, well deve­loped, slender, springing from the disk with a gradual taper, which is continued to the extremity; the upper surface of the ray arched rather than rounded. Interbrachial angles well rounded; the lesser radius is in the proportion of 41.6 per cent.; R=24 millim., r=10 millim. Dorsal surface of the disk slightly inflated above the level of the marginal plates, and with a promi­nent conical peak in the centre of the area.

Dorsal area covered with a thick integument, uniformly beset with well-spaced pseudo-paxillae, which are very small and regular, each carrying 3 or 4 spinelets, those with the latter number being by far the most numerous. The paxillae do not extend along the rays, but are confined to the actual disk-area; a blank space is thus left at the base of the rays, which has the appearance of being closely plated with small round scales im­bedded in the integument. In the neighbourhood of the conical peak the paxillae become very small and crowded. Around this as a centre and at some little distance away, a number of larger paxillae made up of more spinelets are arranged; these are con­gregated with more or less regularity into round groups, of which, roughly speaking, there is one opposite the median line of each ray, with a smaller group intermediate between each of the larger ones. The larger groups consist of ten to twelve large paxillae of about ten spinelets each; and the smaller groups of about five or six paxillae. Outside this conspicuous ring of the disk there are a few large paxillae placed here and there amongst the general small or pseudo-paxillae of the disk.

Marginal plates, instead of forming perpendicular rounded sides, are inclined inwards towards the centre, which gives a bevelled edge to the disk and an arched rather than a rounded character to the upper surface of the rays. The supero-marginal plates do not meet in the median line of the ray, but leave a rather wide suture along the whole length, which expands on approaching the disk. All the marginal plates are longer than high, excepting perhaps the penultimate superior. The superior series are ten in number exclusive of the terminal, and vary in depth very slightly from the arm-angle to the extremity of the ray. The inferior series correspond in number and breadth with the superior series, but diminish gradually in height as they pro­ceed along the ray. The surface of the plates is perfectly smooth, and forms an even contour-line to the ray, the sutures being hardly discernible except with magnifying-power. None of the supero­marginal plates bear spines except the terminal. This plate is comparatively small and inconspicuous, subtriangular in contour, and upturned at a sharp angle from the plane of the ray, a posi­tion that gives a very marked character. It bears three rather short robust spines—one, which is somewhat the stoutest, is placed in the median dorsal line and directed vertically upwards; the other two stand at the anterior ventral angles of the plate, and are directed outward and at an angle of about 45° to the single spine. In consequence of the thinning-off of the terminal plate, the bases of these lateral spines are not far removed from that of the dorsal spine; a deep indentation or sinus occurs between them, in which the ambulacral furrow terminates. Cribriform organs five in each angle, rather wide, and leave only a small band of the plate between adjacent organs; each with a depres­sion down the median line; structure papilliform.

Ambulacral furrows deep and contracted; the adambulacral plates arching considerably over, and the ambulacral spinelets covering-in the area when disposed for that purpose. The adambulacral plates are elongate and subcrescentiform, and each forms an angular prominence on the sides of the furrow, the angles separating to a certain extent the sucker-feet of neighbouring segments. Ambulacral spines four to each plate, short, thin and compressed, uniform in breadth throughout and rounded at the extremity, arranged in a straight, or sometimes slightly curved, line and at a very slight angle to the furrow, the direction of the line being outward from the furrow. A secondary row of 5 or 6 small granules stands on the outer margin of the adambu­lacral plates behind the furrow-series, placed in a slightly curved line; and these become more or less indistinct along the outer portion of the ray.

The mouth-plates are large, prominent, and elevated along the line of suture; the junction is imperfect, and the aboral extremi­ties of the plates being widely open, expose the odontophore. Mouth-spines 6 to 8 on each side, short, compressed and pointed: the

innermost one on each side larger and longer than the rest, and directed towards the actinostome; the lateral ones falling into the furrow and interlocking with the corresponding denticles of the neighbouring mouth-angle. A number (varying from 6 to 12) of small granule-like tubercles are present on the superficies of each plate; two, which are slightly largest, stand near the inner third of the plate; whilst the remainder are con­fined to the aboral half of the plate, and are sometimes arranged in two or three lines and sometimes irregularly. It seems scarcely possible to rank these as secondary mouth-spines; and yet there can be little doubt that they are rudimentary or aborted representatives of these appendages.

The ventral interbrachial areas are triangular in outline, and covered with a regular plating of hexagonal imbricating scale-like plates arranged in columnar series extending from the margin to the furrow parallel with the median interbrachial line. The plates are broader than long, the disproportion increasing as they approach the margin; they bear a few (3 to 5) small, widely-spaced, irregularly disposed granules, some plates here and there having none.

Colour, in alcohol—margins and actinal area greyish white; dorsal membrane bluish grey, with touches of light brown near the margin of the area and occasionally on the groups of paxillae round the centre of the disk.

Station 299. Lat. 33° 31' S., long. 74° 43' W. Depth 2160 fms.; bottom temperature 1.1° C.; grey mud.”

(Sladen, 1883: 237-239)