Ctenamphiura Verrill, 1899.

Ctenamphiura dawbini Fell, 1952 (Figures 11 to 14)

• Dimensions: R, 60 mm.; r, 6 mm.; ratio R/r, 10.

Disc: rounded-pentagonal, constricted at the interradii, tumid, robust Aboral surface covered by a coarse, uneven mosaic of lumpy, thick plates, in the manner of cobblestones, none imbricating, and of smaller size in the interradii. No primary plates. Radial shields longer than broad, pearseed-shaped, separated by a wedge of about six mosaic plates; extending from near the periphery to about one-third of the distance to the centre. Adoral surface of the disc naked, save at the sub-ambital border, and save for scales bordering the genital clefts. The subambital plates are uneven, thick, and lumpy like those above. The genital clefts extend to the fifth arm-segment. The oral shields are large, spearhead-shaped, longer than broad, with an obtuse proximal angle; their distal borders abut upon the outer genital scales, their lateral borders rest upon the first lateral arm-plates, their proximal borders alone adjoin the adoral plates. Adoral plates triangular, small, meeting neither within nor without. Three pairs of oral papillae; an infradental pair, flattened, scale-like, large, placed vertically upon the torus, and hence viewed end-on and so appearing smaller than in reality; an apparent outer pair, which, however, are really an intermediate pair, scale-like, very large, rounded, carried by the adoral plates; and an outermost spiniform pair, long, carried on the adoral plates, but reflexed within the oral cleft so as to be partly hidden by the large and apparent outer papillae.

Arms: robust, compressed, densely invested by erect arm-spines. Upper arm-plates small, fan-shaped, with a proximal angle and a distal convex border, contiguous—largely obscured by the uppermost arm-spines which curve over to meet in the midline. Lateral arm-plates meeting neither above nor below, each provided with eleven or twelve erect, stout arm-spines arranged in a continuous comb. As the arm is unusually robust, the segments are much broader than long, and consequently the arm-combs of succeeding segments are crowded together to form a dense bristling armature. The arm-combs of those segments which underlie the disc are smaller, and comprise fewer spines; the number of the latter decreases from without inwards till at the first segment there are only two spines. The number of spines in the combs at the distal ends of the arms, on the other hand, remain fairly constant almost to the extremities. Lower arm-plates subquadrate, with slightly concave distal borders, broadly contiguous. Tentacle-pores large, page 21

Ctenamphiura dawbini sp. nov.

Fig. 11.—Proximal half of arm viewed obliquely, mainly from below. Fig. 12.—Fig. 13.—Portion of arm, viewed from above at a point where the arm is downwardly flexed, exposing the arm-plates more fully than in Fig. 12. Fig. 14.—Adoral view.

Fig. 11 to upper scale; Figs. 12 and 13 to scale at left; Fig. 14 to lower scale.

Abbreviations: 1, first, or infradental, oral papilla, 2, second, or apparent, outer papilla. 3, third, or true, outer papilla, reflexed below second papilla. A, adoral plate. AM, ambu-lacral vertebra. D, upper arm-plate. G, genital cleft. GS, outer genital scale. L, lateral arm-plate. M, madreporite. N, naked adoral interradial region. O, oral shield. PL, orel plate, S, radial shield. SP, arm-spine. T, tooth. TE, tentacle-scale. V, lower arm-plate.

page 22elongate, almost as long as the arm-segments, with one large, elongate, flattened tentacle-scale borne by the lateral plate. The tentacle-scale of the first segment is rounded, that of the second bean-shaped, that of the third and succeeding ones elongate, quadrangular, twice as long as broad, broadest at the free distal end; so large as to resemble a flattened arm-spine.

Colour: mottled fawn, brown, and grey; the largest oral papillae brown, the arms whitish below.

Type locality: Alert station 2, Pelorus Sound, 25 to 30 fathoms, shell detritus and mud, December 26, 1951; coll. W. H. Dawbin; three specimens.

Holotype: in the Zoology Museum, Victoria University College.

Ctenamphiura has, till now, been a monotypic genus known only from two specimens of C. maxima (Lyman, 1879) which were taken from 28 fathoms at Challenger station 188, western end of Torres Strait. No specimens had been reported since; accordingly, the finding of three specimens of the genus, representing an undescribed species of it, was a great surprise and a notable result of the Alert investigations.

In a preliminary announcement of the rediscovery of the genus (Fell, 1952a), it was stated, inter alia, that Ctenamphiura is characterized by its large outer oral papillae which exceed in size the inner papillae. On subsequent study, I found the true relations to be as stated above and as illustrated in Fig. 14—where the infradental papillae have been drawn overturned to show their real size and the peculiar reflexed condition of the true outer papillae indicated.

C. dawbini and the genotype, C. maxima, share in common not only the generic characters as defined by Verrill (1899)—that is, the characteristic structure and arrangement of the jaws and accessory plates and oral shields—but also the fact that the arm-spines are arranged to form combs. Since it is obvious that Verrill had these combs in mind when he erected the genus, it seems strange that he omitted to note their presence when defining it; the diagnosis clearly should include the character. The two species agree also in the large size of their tentacle scales and the lumpy character of the disc-plates. C. dawbini is distinguished from the congener by having only one (instead of two overlapping) tentacle-scale, and by the partly naked character of the underside; also by the reflexion of the spiniform oral papillae.