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“HYPHALASTER INERMIS, n. sp.
Marginal contour stellato-pentagonoid. Rays five, well developed, slender, round, and tapering but slightly. Interbrachial angles very wide and expansive, the curve slightly flattened in the immediate angle, thereby emphasizing the marked pentagonal contour of the body-disk. The lesser radius is in the proportion of 42.5 per cent.; R=20 millim., r=8.5 millim. Disk depressed, not inflated; both dorsal and actinal surfaces stand on a level with the edges of the marginal plates.
Dorsal area covered with closely crowded the whole disk as well as the base of the rays being uniformly packed. The paxillae are very fine and small, and are made up of about 5 to 10 spinelets; towards the margin of the disk they become smaller and also in the centre, where they are very compact, a slightly prominent peak being formed as in Ctenodiscus. A slight elevation of the surface is present in the median radial line, opposite the base of each ray, and at about one third of the distance from the margin to the centre.
Marginal plates occupy the entire margin and represent the whole thickness of the animal, forming perpendicular walls regularly rounded above and below. Along the rays the supero-marginal plates meet in the median dorsal line and form a complete casing to the ray, which is well rounded, small, and tapers but slightly. The supero-marginal series are 8 in number (or, with a very small aborted one, 9), exclusive of the terminal. The plates which fall in the margin of the disk proper have the length about equal to their height, but in those along the ray the height is greater than the length. The infero-marginal plates correspond in number and in length with the superior series. In the arm-angle, along the disk proper, the height is about equal to the length and the plates are uniform in size with the superior series; towards the extremity of the ray the height diminishes gradually and the length is greater than the height—a reversal of the relative proportions presented by the plates of the superior series. The marginal plates are smooth and bear no spines, but when examined microscopically have the appearance of being subgranular and built up of a rather open network. The plates of both series are convex outwardly or tumid in a very slight degree, by which means the sutural divisions of the segments are clearly marked out, and a somewhat annulated appearance is given to the ray. The terminal plate is large and conspicuous, appearing somewhat tubercular and directed slightly upwards when viewed in profile, and oval in contour when seen from above. This plate bears three short and rather robust spinelets—one at the terminal extremity of the plate, situated in the median dorsal line, pointing in the direction of the prolongation of the ray, and diverging but little from the horizontal. Below this spine, and at either side of it, at the angle formed by the ventral edge of the plate and the terminal extremity, is a somewhat smaller spinelet, pointing in the direction of the prolongation of the ventral margin of the plate. Cribriform organs 7 in number, narrow and well defined; structure papilliform.
Ambulacral furrows narrow and straight, almost completely closed-in by the overarching adambulacral plates and spines, the sucker-feet, which are arranged in simple pairs, being entirely concealed from view. The adambulacral plates are about half as broad as long, but diminish in size as they proceed outwards; and form along the ray triangular prominences projecting into the furrow. Each plate bears 3 to 4 spines, rather short, rapidly pointed, more or less compressed, invested with membrane, arranged in line along the furrow-margin of the plate and sometimes slightly oblique to the course of the furrow. The row of spinelets can be raised at a right angle to the surface of the plate, so as to allow the sucker-feet to be protruded. Traces of an aborted secondary or external spinelet, represented by a mere granule, may be detected at the adoral extremity of the adambulacral plate, away from the furrow-series.
Mouth-plates moderately large, the inner margins which fall in the median suture being elevated so as to form a rounded elongate tubercular protuberance, the lateral margins being flattened out. Mouth-spines 7 or 8 on each side, similar to the ambulacral spines, excepting the innermost one, which is much larger and stouter. Two large spines are thus conspicuous at each mouth-angle and are directed towards the centre, the series entirely closing the peristome, which is remarkably small. The small mouth-spines upon the margin of the plate interlock with those of the adjacent mouth-angle, and form a continuous series with the ambulacral spines. The rudiments of a secondary mouth-spine, represented by a thorn-like granule, occur on each plate near the median suture and at the highest portion of the keel.
The interbrachial areas are triangular in outline, flat, extensive, and covered with imbricating scales of more or less regularly symmetrical hexagonal form. These plates are broader than long, and arranged in regular series of single columns extending from the margin of the disk to the ambulacral furrow; their breadth diminishes somewhat as they approach the margin, and consequently that of the column also. The adambulacral plates join up to the infero-marginal plates along the whole length of the free portion of the ray, and there is consequently no extension of the interbrachial area along the ray. The imbricating plates bear a few widely-spaced miliary tubercles or large granules upon their surface, usually 4 or 5 to a plate, but upon which they have no definite arrangement.
Colour, in alcohol, grey, the paxillar area being a much darker shade, which shows a strong contrast with the greyish white of the marginal plates.
Station 237. Lat. 34° 37' N., long. 140° 32' E. Depth 1875 fms.; bottom temperature 1.7° C.; mud.”
(Sladen, 1883: 239-242)