Arxellia helicoides, Vilvens, Claude, Williams, Suzanne T. & Herbert, David G., 2014

Vilvens, Claude, Williams, Suzanne T. & Herbert, David G., 2014, New genus Arxellia with new species of Solariellidae (Gastropoda: Trochoidea) from New Caledonia, Papua New Guinea, Philippines, Western Australia, Vanuatu and Tonga, Zootaxa 3826 (1), pp. 255-281 : 269-272

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.

Figures

FIGURES 1 – 9. Arxellia new species, all holotypes except A. tenorioi. 1, A. trochos, n. sp., type species of Arxellia n. gen. 2, A. boucheti n. sp. 3, A. herosae n. sp. 4, A. helicoides n. sp. 5, A. tenorioi n. sp. 6, A. tracheia n. sp. 7, A. maestratii n. sp. 8, A. thaumasta n. sp. 9, A. erythrea n. sp. Scale bar = 5 mm.

FIGURES 63 – 69. Arxellia helicoides n. sp., Papua New Guinea, southern seamount of Manus Island, 305 – 579 m [BIOPAPUA, stn DW 3687]. 63 – 65, holotype MNHN IM- 2009 - 15188, 8.2 x 6.3. 66 – 69, paratype MNHN IM- 2009 - 15189. 66 – 67, shell, 9.0 x 6.9 mm. 68, details of protoconch and early whorls. 69, details of columella, umbilicus and aperture. Scale bars = 5 mm, except fig. 68: 100 µm, fig. 69: 1 mm.

Tables

TABLE 5. Arxellia helicoides n. sp.: shell dimensions for type specimens.

	TW	H	W	HA	H/W	H/HA H/TW
holotype MNHN IM-2009-15188	6.4	8.2	6.3	1.8	1.30	4.56 1.28
paratype MNHN IM-2009-15189	6.7	9.0	6.9	2.3	1.30	3.91 1.34
paratype NHMUK 20140009	6.8	9.1	7.2	2.1	1.26	4.33 1.34