“Chitonaster cataphractus, n. sp. (Pl. LXXIII., figs. 1-3).
Rays five. R= 16.5 mm.; r = 5 mm. R>3 r. Breadth of a ray between the second and third infero-marginal plates, 4.75 mm.
General form stellate. Rays well produced and tapering from the base to the extremity. Abactinal area of the disk convex and almost hemispherical as seen in profile, that of the rays rounded and forming with the lateral wall a uniform curvature. Actinal area subplane.
The abactinal area is more or less rigid and entirely covered with closely-fitting hexagonal, or perhaps in places partially rounded, plates overlaid with a uniform layer of membrane, which renders it difficult to define exactly the outline of all the plates. Each plate bears one, and occasionally two or even three, short, robust, cylindrical, obtuse spinelets or elongate tubercles, uniform and equal throughout, the membrane mounting the base, but becoming so thin on the upper part that it is scarcely perceptible. No definite order of arrangement of the plates is distinguishable, but on the outer part of the ray only a single series of abactinal plates separates the supero-marginal plates of the two sides of a ray; and these extend uninterruptedly to the tip and are comparatively large. Occasionally a small granule is present on the plates, usually near the edge. I have failed to detect the presence of any trace of papulae.
The supero-marginal plates, which are thirteen in number, counting from the median interradial line to the extremity, have the height (or breadth) greater than the length, and are slightly arched upon the abactinal surface in conformity with the rotundity of the ray. Each plate bears normally two short, obtuse, cylindrical spinelets or tubercles, similar to those on the abactinal plates, placed one above the other, that is to say along the median line of height at right angles to the direction of the axis of the ray. Sometimes only one spinelet is present, and this usually occurs in the interbrachial arc; and sometimes a much smaller tubercle or granule may be present in addition to the two large ones above described, and this may stand either by the side of the upper spinelet or above it, in the latter case forming a lineal series of three along the median transverse line of the plate.
The infero-marginal plates are similar to the superior series but are not all regularly correspondent, and their breadth is rather greater than that of the latter plates. Each plate bears two or three spinelets or elongate tubercles, similar to those above described, placed in lineal series along the median transverse line of the plate; often one or two of these tubercles are much smaller. At the extremity of the ray the infero-marginal plates are smaller than the superior series, and only bear one spinelet. The surface of the marginal plates is slightly convex in the median transverse line, and the character is more or less emphasised by the membrane mounting the base of the spinelets.
The adambulacral plates are large and broader than long. Their armature consists of three large, cylindrical, obtuse spinelets, which form a transverse series along the median line of the plate at right angles to the furrow. The spinelet on the furrow margin and the middle one are subequal and much longer than any of the other spinelets on the animal; the outer spinelet is smaller. These spinelets radiate slightly apart. Occasionally there is a second and smaller spinelet on the furrow margin on the adoral side of the furrow spine proper, and placed somewhat obliquely in relation to it. Occasionally there may be an additional small spinelet at the outer end of the transverse aeries.
The mouth-plates are small and present no actinal prominence. Their armature consists of three spinelets on the free margin. The innermost spine is large and cylindrical, the second is smaller and slightly tapering, the third is larger than the second and cylindrical, and should perhaps be counted with the superficial series. On the actinal surface of the plate are two much smaller spinelets, the outermost smallest, forming with the outer marginal spine a line of three, corresponding to the three spines on the adambulacral plates. No other spines are present on the plates. The plates do not appear to meet in the median suture, but no superficial indication of this is present, as the whole surface is covered with the same uniform layer of membrane as the other parts of the starfish.
The actinal interradial areas are very small. Not more than four or six intermediate plates are present in each, and each of these bears a small tubercle similar to, but smaller than those on the abactinal plates. Consequently all except the innermost three or four adambulacral plates are contingent on the infero-marginal plates.
The anal aperture, which is subcentral and slightly ezcentric, is conspicuous and margined by a number of robust elongate granules.
The madreporiform body is small and has very coarse striae, which, in consequence of their disposition, give the organ almost the appearance of a group of coarse granules. It is situated nearer the margin than the centre of the disk, at the top of the lateral wall in the interbrachial arc, and is surrounded by the spinelets borne on the adjacent abactinal plates. No pedicellariae of any kind are present.
The ambulacral tube feet have large fleshy disks.
Colour in alcohol, an ashy grey.
Locality.—Station 156. In the neighbourhood of the pack ice, near the Antarctic Circle. February 26, 1874. Lat. 62' 26' 0" S.; long. 95' 44' 0" E. Depth 1975 fathoms. Diatom ooze. Bottom temperature 32.1° Fahr.; surface temperature 37.2° Fahr.”
(Sladen, 1889: 284-285)
