Amphioplus Verrill, 1899

Amphioplus longirima sp. nov. (Figures 8 to 10)

Dimensions: r, 2·5 mm., the arms broken in the type, but evidently exceeding the minor radius in length by ca. seven times.

Disc: pentagonal, constricted at the interradii. The aboral surface is covered by imbricating scales which are larger at the centre than at the periphery. A single

Amphioplus longirima sp. nov.

Fig. 8.—Aboral view. Fig, 9.—Adoral view. Fig, 10.—Lateral view of arm-segment. All to scale shown.

Abbreviations: A, adoral plate. D, upper arm-plate. DC, dorso-central primary plate of disc G, genital cleft. L, lateral arm-plate. O, oral shield. S, radial shield. V, lower arm-plate.

page 19 dorso-central primary plate is distinguishable. Radial shields reaching to only about one-third of the distance from the ambitus to the centre, more or less bean-shaped, almost contiguous distally but divergent proximally at an acute angle; the intervening angle is filled in by about four polygonal plates. The inter-radial borders of the radial shields are overlapped by the neighbouring disc-scales. Adoral surface completely scaled, the scalation finer than on the aboral surface. Genital clefts extending to the fourth arm-segment. Oral shields of the spearhead-shaped type, but modified, the inner extremity (i.e., the "point" of the spearhead) being rounded, not angular. The oral shields are longer than broad. The adoral plates are triangular, meeting within. There are four pairs of oral papillae, one pair infradental, one pair carried by the oral plates, and the two outer pairs are borne by the adoral plates. All the oral papillae are leaf-shaped, but the outermost have pointed tips, and evidently are really of the spiniform type.

Arms: upper arm-plates broader than long, subtriangular, with a broad distal convex border which overlaps the proximal angle of the succeeding plate, save in the first upper plate, where the proximal obtuse angle is exposed. Lateral plates meet neither above nor below, save in the case of the segment which bears the first upper arm-plate (segment five when counted from below); in this case the lateral plates meet in the midline above. Lateral plates each bearing on the basal arm-segments four similar, stout, short spines; on the more distal segments, three spines only. Lower arm-plates subquadrate, with concave distal borders and lateral borders emarginated by the tentacle-pores; neighbouring plates contiguous over most of their width. Tentacle-pores large, having two similar leaf-shaped tentacle-scales on the inner side, both scales being carried, therefore, by the corresponding lower arm-plate.

Colour, in spirit, white.

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

Holotype: the unique specimen is in the Zoology Museum, Victoria University College.

This, only the second species of Amphioplus to be discovered in New Zealand waters, appears to be more closely related to its New Zealand congener A. basilicus, than to any other Pacific or Antarctic form. It can, however, be easily distinguished from A. basilicus by the following: (1) the long genital clefts (to which the specific name refers)—these reach to near the ambitus, whereas in A. basilicus they do not pass beyond the first arm-segment; (2) the oral shields are spearhead-shaped, whereas they are triangular in A. basilicus; (3) the radial shields are somewhat larger and more widely separated. Of the six known Australian species, only A. stemspis and A. didymus seem at all comparable; but the former is distinguished by its long, slender, radial shields, the latter by its larger inner tentacle-scale.