Ophiura Lamarck, 1816, emend. Matsumoto

Ophiura chathamensis sp. nov. (Figures 15 to 20)

Dimensions: minor radius, 3 mm.; arms broken, but probably three times minor radius.

Disc, circular, flattened. Aboral surface covered by imbricating, thin, closely adpressed scales of varying size; the larger ones occur at the centre, at the ambitus, in the interradii, and between the radial shields. Primary plates indistinct. Radial shields pearseed-shaped, distally contiguous, separated proximally by a wedge of six or eight polygonal plates, and extending less than half-way to the centre of the disc. Adoral surface completely sealed by closely imbricating uniform plates. Genital clefts extending from the oral shields to the upper part of the arm-base, with genital papillae forming the interradial border. Oral shield large, broadly pentagonal, with a convex distal base, concave sides, and an obtuse proximal angle. Adoral plates small, broader than long, meeting broadly within, contiguous only with the proximal border of the oral shield. Oral plates larger than the adoral plates, with three pairs of oral papillae.

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Arms: provided with basal arm-combs, each carrying about ten papillae. Upper arm-plates contiguous on the proximal four segments, thereafter isolated, their shape changing from subquadrate at the base to longitudinally ovate at the fourth plate and beyond, the narrow end of the ovate plates being proximal. Lateral plates meeting both above and below, save on the segments bearing the first four upper arm-plates, where the lateral plates meet below only. They bear three slender, pointed arm-spines, of which the uppermost is longest, equal in length to one segment, the lowermost smallest. Lower arm-plates small, not contiguous, mostly fan-shaped with a proximal obtuse angle and convex distal border; but those of the second and third arm-segments are transversely rhomboid, and that of the first segment longer than broad. Second oral tentacle-pores large, opening into the oral clefts, each ringed by nine or ten small scales. The first three or four lateral plates of the arm carry each a tentacle-pore, the first one large with four or five scales, the second of medium size with three scales, the third (and fourth, if present) minute with one or no scale; beyond, there are no pores.

Colour, in spirit, white.

Type locality: Discovery station 2733, Chatham Rise, west of Chatham Islands, 30 metres, November 4, 1950; one specimen.

Holotype: Zoology Museum, Victoria University College.

From Ophiura rugosa, the only other species of Ophiura known from New Zealand, O. chathamensis is sharply distinguished. The former has large, rounded aboral plates arranged in a symmetrical pattern, the proximal under arm-plates are contiguous, and the arm-spines are very short. O. chathamensis shares with Ophiocten hastatum, a generic relative, the character of having slender spines in groups of three, the uppermost elongate; but it is distinguished by lacking the uniformly fine scaling of the aboral surface and by the larger and differently shaped radial shields. Benham's Ophiura kermadecensis is now referred to Amphio-phiura, and is not similar to the present species.

Of the Pacific species of Ophiura, O. chathamensis appears to be more nearly related to Ophiura ooplax (H. L. Clark) than to any other—a species which ranges from Japan to South Australia. The resemblance is most marked in the form of the radial shield, the general structure of the arms, and the three slender arm-spines. The differences, however, are considerable: the arm-combs are lost in the adult O. ooplax, where, also, all the arm-spines are elongate. In O. chathamensis, there are three oral papillae (eight or nine in O. ooplax), and the tentacle-pores decrease in size more rapidly than they do in the latter species. O. Chathamensis also has a finer aboral scaling than O. ooplax.

The ophiuroid illustrated (Figs, 18 to 20) was taken with this species at Discovery station 2733. Despite the large oral shields, which suggest Amphiophiura, it is clearly an Ophiura, as the rapid declension of the tentacle-pores indicates. Although the aboral surface of the disc is markedly different, and there are only page 28 two arm-spines, it exhibits considerable resemblance to O. chathamensis, of which it might be a juvenile stage. I therefore refrain from doing more than illustrate its structure.