“HYMENASTER CAELATUS, n. sp.
Marginal contour stellato-pentagonoid; interradial angles sharply indented, the lesser radius being in the proportion of 63.3 per cent.; R=30 millim., r=19 millim. The dorsal area is concave, the radii being curved upward and their extremities recurved and quite turned over; actinal area very convex. The radial areas are well marked out, the lateral margins converging gradually to the tip, which is not attenuated or produced. A secondary membrane extending beyond the actino-lateral spines forms a conspicuous fleshy fringe.
Supradorsal membrane rather thick. Paxillae with 4 to 5 spinelets, forming regular crowns, which elevate the membrane into rhomboid or pentagonal areas, raised in relief as it were, and closely placed, the margins of the tabula being more or less incurved, and the interspaces between deep and furrow-like. Muscular fibres closely interwoven, bands ill-defined, and meshes irregular; spiracula small, and frequently two or more together—this structure being uniform over elevated areas and interspaces alike. Tips of the paxillae-spinelets only slightly protuberant; the relief-areas which fall in the margin of the ray are smaller, more compressed, and somewhat modified in form from the others. About 7 longitudinal rows of' elevated areas or tabulae may be counted at the base of a ray.
Oscular orifice small, circumference at the base of the valves pentagonal, 9.25 millim. in diameter, and marked out by spinelets. Valves 5, very regularly triangular, apices sharply pointed, all webbed together, the whole forming a regular pyramid when closed. The two innermost spines of each valve stand somewhat apart, and the membrane is rather deeply drawn in between, producing a well-defined furrow along the median line. The projecting tips of spinelets are prominent at the sides of the valves. At the base of each valve are two large elevated areas, formed by paxillae-crowns of 6-8 spinelets, and thus nearly twice the size of the other elevated tabula upon the dorsal surface: they are subpentagonal in shape; and the ten form a conspicuous and well-defined circlet round the oscular pyramid.
Ambulacral furrows rather wide, straight, uniform in breadth, except at the extremity, where they rapidly converge. Ambulacral spines 3, very short, cylindrical, slightly tapering, covered with membrane, placed in line oblique to the furrow. Aperturepapillae large and subcircular, with its investing membrane somewhat Japanese fan-shaped. The calcareous portion of the papilla is very flaring in habit, sometimes appearing as if made up of a comb of radiating spinelets.
Mouth-plates small, short but broad, with widely-expanded lateral flanges, broadly rounded in front, keel along line of junction feebly developed, aboral extremity only slightly prominent. Two secondary surface-spines borne on slight tubercles, one near the adoral extremity, and the other near the middle of the plate. These spines are short, comparatively small, broad at base, and taper slightly at the tip—the adoral pair being rather the smaller, and not much larger than the ambulacral spines. Mouth-spines 3, similar in size and form to the ambulacral spines, arranged on the lateral margin of the plate and away from the adoral peak of the keel.
Actino-lateral spines widely spaced, the difference in the length being comparatively small along the inner two thirds of the ray. About 27 spines on each side of a ray, the 6th or 7th from the mouth slightly longest. None of the spines meet in the interbrachial median line, but are widely separate; they are covered with a thick fleshy tissue, which is slightly turned over the tips of those spinelets that fall beyond the arm-angle, and is then extended in the form of a fleshy supplementary web or fringe, which is very conspicuous in the arm-angle, and extends up to within a short distance of the extremity of the ray, gradually diminishing in breadth as it proceeds outwards. The margin of this supplementary fringe is thickened, and furnished with a powerful muscular band.
Colour, in alcohol, a rather livid pink; ambulacral furrows and sucker-feet a yellowish white.
Station 158. Lat. 50° 1' S., long. 123° 4' E. Depth 1800 fms.; bottom temperature 0.3° C.; Globigerina-ooze.”
(Sladen, 1882: 225-227)
