Clade A, Williams et al., 2013
Type species: Arxellia trochos n. sp. —Recent, New Caledonia.
Diagnosis. Shell small to moderate (height up to 11.8 mm, width up to 13.0 mm), more or less conical in shape, spire moderate to high. Protoconch diameter 300–350 µm, with up to 6 thin spiral threads (in one species only abapical threads visible) and a thin, straight terminal lip without varix. Early teleoconch whorls usually with 5 or 6 beaded spiral cords of equal strength; adapical cord P 1 rapidly strengthening and becoming beaded; one abapical primary cord (P 5 or P 4 according to species) forming a low suprasutural carina starting from third whorl; carina may weaken or disappear on last whorl. Peristome incomplete; columella thickened in some species, with callus lobes at base and mid-point; granular deposition of calcium carbonate occurs where columella is thickened. Base convex to almost flat, with beaded cords on whole surface or with an intermediate smooth area, but always with at least one strong beaded spiral cord bordering umbilicus. Umbilicus open, narrow to moderately wide. The pink, green and opalescent shell colour of most species is larely due to a nacreous shell layer, rather than pigment. However, in some species shells do appear to be pigmented (e.g. the brown colour in A. boucheti, A. herosae, A. helicoides, A. maestrati and A. erythrea).
Radula of type species with well-developed latero-marginal plates; formula (5–6)+ 1 + 4 + 1 + 4 + 1 +(5–6).
External anatomy typical of family; eyes pigmented (where known; see Williams et al. 2013); left neck lobe in the form of two short, non-papillate tentacles, the right one a trigonal lobe just behind eye-stalk; three papillate epipodial tentacles of more or less equal size present on each side, and additional smaller tentacle-like projection between the anterior pair on both sides. Operculum corneous, straw-brown, shallowly concave, multispiral with central nucleus; growing margin short.
Remarks. Diagnosis of the new genus is based on the nine species discussed herein. Live specimens collected at 242– 650 m.
Discussion. The new genus Arxellia corresponds to "Clade A" in the recent molecular phylogeny of the Solariellidae published by Williams et al. (2013). Seven of the new species and many of the specimens photographed were included in that study. The type species was not included, but unpublished molecular data confirm that it belongs in this clade (Williams, unpub. data). Genetic studies place Arxellia as the sister taxon to Archiminolia Iredale, 1929 with strong support. The two genera can be separated by shell shape (Archiminolia species have turban-shaped shells whereas Arxellia species have conical shells) and by the ontogeny of teleoconch spiral cords. In Arxellia there are numerous beaded spiral cords and a suprasutural carina on the intermediate whorls, whereas in Archiminolia there are at most a few subsutural cords on these whorls.
Arxellia and Archiminolia form a well supported clade with Spectamen Iredale, 1924, Solariella Wood, 1842, Bathymophila Dall, 1881, Minolia A. Adams, 1860, Hazuregyra Shikama, 1962, Zetela Finlay, 1926 and an as yet undescribed genus (Clade B) in the molecular tree (Williams et al. 2013). Although relationships between genera within this larger clade are not well resolved, the first five genera comprise a poorly-supported clade in the MrBayes tree. The presence of well-developed latero-marginal plates in the radula of Arxellia is consistent with the radula morphology of other members of this latter clade, such plates having been reported in Bathymophila, Spectamen, Archiminolia and at least some species of Solariella (Herbert 1987; Warén 1993; Marshall 1999). In other solariellid genera outside this clade, namely Ilanga Herbert, 1967, Clade C (new genus, Vilvens & Williams 2014) and Minolia but also Zetela, this plate is either not present or only weakly developed, retaining a flimsy shaft and cusp. The presence of strong, elongate-rectangular latero-marginal plates may thus be an apomorphic character state shared by most genera comprising this broader clade.
Etymology. Stronghold, citadel (Latin: arx)—referring to the strong columella of some species of the new genus (feminine).
FIGURE 10: Approximate locations of MNHN and WAM expeditions discussed in this paper: ★: PANGLAO 2005; DZ: BIOPAPUA; D MUSORSTOM 8, VOLSMAR; ⊙: BORDAU 1; r: BORDAU 2; ♠: BIOCAL, MUSORSTOM 6, VOLSMAR, SMIB 8, BATHUS 1, BATHUS 2, LITHIST, NORFOLK 1, NORFOLK 2, EXBODI; DZ: CHALCAL 2, SMIB 8; ®: MUSORSTOM 7; + 'Southern Surveyor'.
Key to Arxellia species (based on conchological features).
1. Later whorls with at least 3 beaded or subgranular spiral cords................................................. 2
- Later whorls smooth......................................................................... A. thaumasta
2. Later whorls mostly covered with beaded or subgranular spiral cords............................................ 3
- Later whorl with only 3, widely spaced, beaded or subgranular spiral cords................................ A. tenorioi
3. Shell much higher than wide................................................................... A. helicoides
- Shell height and width more or less equal.................................................................. 4
4. Shell height attaining 12 mm, periphery more or less rounded.......................................... A. boucheti
- Shell height less than 9 mm............................................................................. 5
5. Columella callus with two lobes extensively covering umbilicus........................................ A. herosae
- No columella callus or expansion slightly covering umbilicus.................................................. 6
6. Peripheral carina on P 5................................................................................ 7
- Peripheral carina on P 4................................................................................ 8
7. Intervals between spiral cords narrower than cords, carina relatively weak................................. A. trochos
- Intervals between spiral cords wider than cords, carina stronger....................................... A. maestratii
8. Intervals between spiral cords wider than cords..................................................... A. tracheia
- Intervals between spiral cords narrower than cords.................................................. A. erythrea