Arxellia helicoides Vilvens, Williams & Herbert, 2014, n. sp.

Arxellia helicoides n. sp.

(Figs 4, 63–69, Table 5)

Clade A, sp. 1— Williams et al., 2013.

Type material. Holotype (8.2 x 6.3 mm) MNHN IM- 2009-15188. Paratypes: 1 MNHN IM- 2009-15189, 1 NHMUK 20140009.

Type locality. Papua New Guinea, southern seamount of Manus Island, BIOPAPUA, stn DW3687, 3°04'S, 147°32'E, 305– 579 m.

Material examined. Papua New Guinea. BIOPAPUA: stn DW3687, 3°04'S, 147°32'E, 305–579 m, 2 lv (holotype and paratype MNHN IM- 2009-15189).—Stn DW3688, 3°04’S, 147°32'E, 402–640 m, 1 lv (paratype NHMUK 20140009).

Distribution. Papua New Guinea, 402–579 m (living).

Description. Shell: Size moderate to large for genus (height up to 9.1 mm, width up to 7.2 mm), conical, higher than wide; height 1.3x width, 3.9–4.6x aperture height; periphery angulate; umbilicus funnel-shaped. Protoconch paucispiral, diameter approx. 300 µm, rounded, smooth adapically, but with crisp granules and two thin, equally spaced, spiral threads on abapical half; terminal lip straight, without varix. Teleoconch of up to 6.8 whorls; first two whorls convex, subsequent whorls almost straight; shoulder horizontal on early whorls, disappearing on later whorls; whorls sculptured by 8–12 spiral cords on last whorl. Suture not canaliculate but clearly visible. First whorl with 5 thin spiral cords arising in middle of whorl, cords of equal size and spacing; P1 marking rim of horizontal shoulder at end of whorl, close-set axial threads clearly visible in intervals at end of whorl. On second whorl, P1 strongest; P3 weakening and almost vanishing at middle of whorl; axial threads fading at end of whorl; prosocline axial folds developing at beginning of whorl, stronger on subsutural area and adapical part, rendering P1 and P2 nodular. On third whorl, P1 and P5 strongest, coarsely beaded; P5 forming weak suprasutural carina; axial folds spanning entire whorl. On fourth whorl, nodules of P1 and P5 adapically oriented; carina on P5 more marked; all cords beaded; P6 emerging from suture, beaded, weaker than other cords; shoulder oblique. On fifth whorl, P2, P3 and P4 stronger; axial folds much weaker; S4 arising, equalling P 4 in strength between end of fourth whorl (holotype) and sixth whorl (paratype 189), beaded. On sixth whorl, P5 carina less prominent; shoulder evanescing; S6 arising on fifth (paratype 189) or sixth (holotype) whorl, subgranular. On last whorl, all cords similar in size, except P1 stronger and P5 slightly stronger; carina on P5 possibly weaker (paratype 189); tertiary cords may appear between S4 and P5 (holotype) or under P5, under P6 or between P3 and P4 (paratype 189). Aperture roundly quadrate, weakly oblique; peristome incomplete; outer and inner lips thin at rim; angle at base of columella weak. Columella vertical, concave, with a small basal flange fused to callus forming one lobe at mid-point, slightly overlapping umbilicus. Base flat, smooth except for 2 outer and 1 inner spiral cords; outer cords thin and smooth, inner cord forming rim of umbilicus, strongly beaded; weak axial threads on intermediate area. Umbilicus wide (diameter ca. 21–26% of shell width), but funnel-shaped and thus narrower internally; rim angulate, sometimes with 2 beaded spiral cords just inside rim (holotype).

Colour: Early teleoconch whorls white, subsequent whorls brownish-grey; base nacreous green, umbilical area white; protoconch white.

Remarks. Arxellia helicoides superficially resembles Lamellitrochus pourtalesi (Clench & Aguayo, 1939) from the tropical western Atlantic, but although of similar size, that species has three strong nodular spiral cords on the last whorl, numerous spiral threads on the base, and a more rounded aperture. Among other Arxellia species, A. helicoides is distinctive on account of its particularly high spire (having the highest H/W ratio of ca. 1.30). In the molecular analysis of Williams et al. (2013) this species forms a clade with A. tracheia and A. boucheti.

Etymology. Helical, helicoid (Greek: ειξ: helix and εδο: figure)—with reference to the generation of the shell, following a mathematical conical helix.