Herpetopoma (s.l.) xeniolum (Melvill, 1918) Herbert, 2012

Herbert, D. G., 2012, A revision of the Chilodontidae (Gastropoda: Vetigastropoda: Seguenzioidea) of southern Africa and the south-western Indian Ocean, African Invertebrates 53 (2), pp. 381-381 : 444-446

Herpetopoma (s.l.) xeniolum (Melvill, 1918) comb. n.

Figs 36–38

Euchelus xeniolum: Melvill 1918: 154 , pl. 5, fig. 27. Type loc.: Chabar , Gulf of Oman [= Chah Bahar, Iran], 5 fath. [- 9 m] (Townsend).

Turcica (Perrinia) waiwailevensis: Ladd 1982: 23 , pl. 24, figs 10–13. Type loc.: Station C 2026, Viti Levu, Fiji; Pliocene. Syn. n.

Herpetopoma eboreum: Vilvens & Héros 2003: 61 , figs 1–4; Poppe et al. 2006: 36, pl. 10, fig. 2; Poppe & Tagaro 2008: 174, pl. 32, fig. 1. Type loc.: Touho Pass, Touho area, New Caledonia, - 50– 62m. Syn. n.

Euchelus townsendianus [non Melvill & Standen, 1903]: Jay 2009.

Type material: Holotype of Euchelus xeniolum Melvill, 1918 in NHMUK (1921.1.28.30); Trew (1987) was in error in stating that there were three specimens registered. Holotype of Turcica (Perrinia) waiwailevensis Ladd, 1982 in USNM (2501420). Holotype of Herpetopoma eboreum Vilvens & Héros, 2003 in MNHN.

Other material examined (all MNHN): RÉUNION: off Ste-Marie (20.867°S 55.633°E), - 110 m, Marion-Dufresne 32, st’n DC126, dredged, 1982; off St-Joseph (21.383°S 55.617°E), - 205–215 m, Marion-Dufresne 32, st’n DR47, dredged, 1982 ; Réunion, not further localised (M. Jay coll’n) . MADAGASCAR: West of Nosy Be (13.450°S 47.917°E), - 187–247 m, Campagne Miriky, st’n DW3234, dredged, 3.vii.2009 .

Remarks:A number of samples collected off Réunion are evidently referable this species, which I consider is closer to Herpetopoma than to Euchelus on account of its small size and the U-shaped notch at the columella base (albeit relatively weak). The species is poorly known and has not to my knowledge been recorded subsequent to the original description. The holotype ( Fig. 37A, B ) has four granular spiral cords on the penultimate whorl as do some Réunion specimens ( Fig. 37C, D ). However, other specimens in the Réunion samples have five such cords ( Fig. 37E, F ) and closely resemble the holotype of Turcica (Perrinia) waiwailevensis Ladd, 1982 from the Pliocene of Fiji ( Fig. 37G, H ). I have no doubt that the Réunion samples belong to a single species and consider the number of spiral cords on the penultimate whorl (four or five) to be a variable character. There is evidently similar variation in the number of spiral cords on the base (6–8). As a consequence, I can find no substantive differences by which to separate Herpetopoma waiwailevensis from H. xeniolum . In addition, H. eboreum Vilvens & Héros, 2003 , described from shallow water off New Caledonia and recorded also from deeper water off the Philippines ( Poppe et al. 2006), appears indistinguishable from H. waiwailevensis . I believe that all three names refer to a single widespread species, for which H. xeniolum is the earliest name. Euchelus hummelincki Moolenbeek & Faber, 1989 from the Caribbean is another similar species, but is evidently smaller (length up to 3.2 mm) and has somewhat coarser sculpture.

The holotype of H. xeniolum was collected in shallow water (- 9 m) and similarly the overall facies of the fauna at the type locality of H. waiwailevensis is that of a shallow-water, lagoonal system ( Ladd 1966, 1982). The depth at which the dredged material from Réunion was obtained (- 110–340 m) would thus seem to be anomalous. However, Réunion has a steeply shelving sublittoral zone and dead specimens of typically shallow-water species are frequently found at greater depths.That the species is also present in the M. Jay collection ( MNHN) suggests that it does indeed occur in relatively shallow water around Réunion. The same may also apply to some rather worn specimens from off the steeply shelving coast of Nosy Be, which I also tentatively identify as H. xeniolum .

This material resembles H.? naokoae (above), but the whorls are less convex and the shell is consistently more elevated, L/D usually>1.23, spire height/aperture height usually> 1.60 in H. xeniolum (usually <1.23 and <1.60 respectively in H.? naokoae ). Additionally, in H.? naokoae the sculpture is coarser and the denticles inside the outer lip, although ridge-like, are restricted to the thickened region just inside the lip edge. In H. xeniolum some of these ridges extend deeply into the aperture. Furthermore, in the local H.? naokoae specimens there is an additional small denticle adjacent to the notch between the two larger denticles at the junction of the base and columella, this is not present in the H. xeniolum material examined.

The protoconch is translucent white with a diameter of ca 220 µm; its surface is eroded in the material available.A microsculpture of irregular vermiform spiral threads is present on the early teleoconch whorls; subsequent whorls with weak oblique vermiform threads and traces of very fine scratch-like marks.

Figures

Fig.36.Distribution of Herpetopoma?naokoae (triangles) and H.xeniolum(squares).Each symbol represents one or more records.

Fig.37.Herpetopoma xeniolum(Melvill, 1918):(A,B) holotype of Euchelus xeniolumMelvill, 1918,length 4.2 mm,diameter 3.0 mm (NHMUK 1921.1.28.30); (C, D) Réunion,length 3.5 mm,diameter 2.4 mm (M. Jay coll’n, MNHN); (E, F) Réunion, Marion-Dufresne 32, st’n DC126, length 3.9 mm, diameter 2.9 mm (MNHN); (G, H) holotype of Turcica (Perrinia) waiwailevensis Ladd, 1982, length 4.4 mm, diameter 3.3 mm (USNM 2501420).

Fig. 38. Herpetopoma xeniolum (Melvill, 1918): (A) shell apex, Réunion, bar = 0.5 mm (M. Jay coll’n, MNHN); (B) adult microsculpture, Réunion, bar = 200 µm (M. Jay coll’n, MNHN); (C) protoconch, off Ste-Marie, Réunion, bar = 100 µm (MNHN).