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“STYRACASTER ARMATUS, n. sp.
Rays five, long and slender, subcylindrical, nearly uniform in thickness throughout their length, the expansion at the base and the attenuation at the tip being very slight. Interbrachial angle very wide, with the curve somewhat flattened in conformity with the pentagonal character of the disk. The lesser radius in the proportion of 28 per cent.; R=38 millim., r=11 millim. Disk depressed, not inflated, and not higher than the marginal plates. Dorsal area covered with a leathery integument beset with minute imperfect pseudo-paxillae and simple spiculate spinelets closely crowded, the general appearance being that of spinelets only. Spaces at the base of the rays naked. No definite epiproctal protuberance, a faint indication of the centre only present.
Marginal plates high, curving inward slightly above and below, and forming a rounded margin. The supero-marginal series are 9 in number from the median interradial line to the extremity, exclusive of the terminal, and all are considerably longer than high. Along the rays, commencing at the fourth plate from the arm-angle, the supero-marginal plates of either side meet in the median dorsal line and entirely encase the upper portion of the ray. The ray is rather compressed laterally, and the dorsal surface is arched. Each alternate supero-marginal plate along the ray bears a large robust conical spine placed in the median line of the ray, the series forming a single line of five spinelets, which stand perpendicular to the ray and diminish in size as they proceed outward. The longest spinelet remaining measures about 6 millim., and the tip appears to have been broken. The penultimate supero-marginal plate is small, and the ray is slightly bent upwards at the extremity. The terminal plate is small and comparatively inconspicuous, not at all swollen or tubercular, and is less than the pair of antepenultimate supero-marginal plates. It bears three spinelets, one in the median line above the extremity of the ambulacral furrow, and two beneath, all close together. The infero-marginal plates correspond in number with the dorsal series, but their length is very much greater in relation to their height—fully three times. The four outermost plates do not correspond in breadth with the companion superomarginal series: Cribriform organs three in number in each angle, rather broad, but well defined; structure papilliform.
Ambulacral furrows very narrow, quite closed-in by the overarching adambulacral plates and spinelets. Adambulacral plates elongate and vertebra-like, the margin to the furrow being deeply scooped out, the extremities prominent and thickened. Ambulacral spines three, moderately long, sharply tapering and slightly compressed: two stand near the adoral extremity of the margin, equal in length, stretching over the furrow, and radiating apart from one another; the third spinelet is slightly smaller, placed about midway on the margin of the adambulacral plate, and is directed in the same direction as the aboral of the two spines. The spinelets are invested with a very fine membrane, which is continuous at their bases, and the spinelets of one side of the furrow interlock with those from the other. Behind the furrow series, at the adoral end of the plate and standing on the prominent swelling, away from the margin of the furrow, a small conical spinelet rises perpendicularly from the surface of the plate; and this becomes more or less rudimentary as it proceeds along the free portion of the ray.
The mouth-plates, which are large, prominent, and not united along the median suture, have a peculiar appearance, resembling the shape of a coulter in a marked degree. The mouth-spines 5 or 6 in number, placed side by side along the lateral margin of the plate, are the same in length and character as the ambulacral spines, and they interlock with the corresponding spines of the neighbouring imbuth-angle. About three small, aborted, tubercular spinelets situated on the surface of the plate are probably the representatives of secondary mouth-spines. The mouth-spines are wide apart and unclosed at their aboral extremity, and expose the odontophore. Actinal interbrachial areas rather large and triangular, covered with squamous ventral plates, which are narrow, elongate, imbricating, and regularly arranged in columns on the outer half of the area, but become larger, broader, sub-rotund and irregular as they approach the mouth-angle. The plates are smooth, without granules, and the investing membrane is of such remarkable thinness that its presence is almost questionable.
Colour, in alcohol, grey, with a slightly brownish shade over the paxillar area.
Station 224. Lat. 7° 45' N., long. 144° 20' E. Depth 1850 fms.; bottom temperature 1.3° C.; Globigerina-ooze.”
(Sladen, 1883: 232-234)