Ophiomastus Lyman, 1878

Ophiomastus stellamaris sp. nov. (Figures 23 to 27)

Dimensions: R, 10·0 mm.; r, 1·5 mm.; ratio R/r, 6·7. The arms are relatively shorter in immature forms.

Disc: circular in outline as seen from above, robust, extremely tumid above, so that the profile is semi-circular. Aboral surface completely covered by imbricating scales, of which the six primaries occupy most of the area in the adult (Fig. 23); in the immature form the six primaries comprise the whole of the aboral surface of the disc (Fig. 26). There is a large pentagonal dorso-central, surrounded by the five primary radials which are of irregular polygonal shape in the page 31 adult (though more nearly pentagonal in the young). Beyond these lie five small, irregularly shaped interradials and five pairs of small, polygonal radial shields. Sundry small angular platelets make up the ambital region and fill in the interstices

Ophiomastus stellamaris sp. nov.

Fig. 23.—Aboral view. Fig. 24.—Lateral view of disc and base of arm, seen from radial aspect, to show its hemispherical tumidity. Fig. 25.—Adoral view. Figs. 26 and 27.—Aboral and oblique lateral views of young individual, showing large primary plates.

Figs. 23 to 25 to scale shown below. Figs. 26 and 27 to scale at right.

Abbreviations: A, adoral plate. D, upper arm-plate. DC, primary dorso-central plate of disc. I, primary interradial plate of disc. L, lateral arm-plate. O, oral shield. OP, oral papillae. R, primary radial plate of disc. S, radial shield. SP, spine of lateral arm-plate. TE, tentacle-scale. V, lower arm-plate.

page 32between the larger plates. Usually, but not always, there is a vertical row of several interradial plates (as in Fig. 24). The radial shields are confined to the edge of the disc, at the insertion of the arms, and are separated from each other by two or three smaller polygonal scales, and from neighbouring shields by the interradial plates. The adoral surface is flat. The oral shields are shield-shaped, longer than broad. The adoral plates are small, longer than broad, meeting each other within, contiguous with the oral shields only along the proximal border of the latter. Oral plates small, but carrying a series of ca. four pairs of prominent, quadrangular, closely-packed oral papillae.

Arms: Upper arm-plates fan-shaped, as long as broad, with a convex distal border, contiguous only in the proximal part of the arms, smaller distally. Lateral arm-plates meeting above and below, except on the basal segments, bearing each a solitary arm-spine on the distal, median border of the lower side. The arm-spine is small but robust, conical, directed distally, not visible from above. The lower arm-plates are contiguous and longer than broad on the proximal two or three segments; beyond, they are not contiguous, but become smaller, and relatively broader than long; they have a prominent convex distal border, and the proximal half of each lateral border is emarginated by the large tentacle-pore. There is a single, large, circular, flattened tentacle-scale, attached to the lateral plate, completely covering the pore.

Colour, in spirit, creamy white.

Type locality; Discovery station 2733. Chatham Rise, 30 metres, November 4, 1950; eleven specimens.

Holotype: Zoology Museum. Victoria University College.

Remarks.—With Ophiomastus tegulitius Lyman, the only species of the genus hitherto known from New Zealand (Cook Strait. 275 fathoms, H.M.S. Challenger), the present species shows no close relationship. The slit-like tentacle-pores, the two tentacle-scales (small and semi-lunar in shape), the paired laterally-borne arm-spines, and the more rapidly tapering arms are some of the distinguishing features of the former. On the other hand, O. stellamaris does show a close resemblance to Ophiomastus secundus Lyman, a species known only from West Indian and central Atlantic seas. The latter species shares with the present one the possession of a single large tentacle-scale, and may have only one arm-spine. However, in O. secundus, two arm-spines occur, even in small individuals, and they are carried, furthermore, on the sides of the arm, not below; also, the oral plates do not carry oral papillae, only a marginal border representing them. In most other respects, the two species are quite similar. From O. perplexus Koehler and O. tumidus Koehler, the species likewise differs in having only one arm-spine.