Channelaxinus

Oliver, P. Graham, 2015, Deep-water Thyasiridae (Mollusca: Bivalvia) from the Oman Margin, Arabian Sea, new species and examples of endemism and cosmopolitanism, Zootaxa 3995 (1), pp. 252-263 : 258

“ Channelaxinus ” investigatoris ( Smith, 1895)

Figs 2 D–E

Material. One fragment, Discovery cruise 211 st. 12685#6, 19°18.88´N 58°15.46´E, 688 m, collected P.G. Oliver, 15 October 1994, NMW.Z. 1995.009.18: 5v, Sri Lanka, Ex Investigator Expedition, no precise locality, Melvill Tomlin Collection, NMW.Z.1955.158.11399.

Description. To 40 mm in length. Robust. Equivalve. Strongly inequilateral, beaks, prosogyrous and close to the anterior. Outline obliquely polygonal. Anterior dorsal margin steeply sloping, straight; anterior very short, narrowly rounded; anterior ventral long and straight, ventral short curved; posterior ventral weakly sinuous; posterior deeply sinuous; posterior dorsal long and straight. Surface multi-contoured, lunule deeply excavated and bounded by a strong ridge, anterior slope very narrow as a flattened ridge; anterior median slope wide, flat; median slope narrow, as a flat ridge; posterior median area weakly sulcate; posterior sulcus very strong; submarginal sulcus sharply defined; auricle weakly developed not rising above submarginal sulcus. Hinge edentulous; ligament in a shallow depression ~two-thirds the length of the submarginal sulcus. Shell chalky white in colour.

Remarks. This distinctive species was collected by the Investigator expedition at three localities off the west coast of Sri Lanka between 260–1114 m ( Smith 1895, 1896, 1906) and at one locality off the east coast of Sri Lanka at 786 m ( Smith 1904). The fragment collected off the Oman margin at 688 m suggests that this species is associated with the hypoxic conditions found in the Arabian Sea.

Figures

FIGURE 2. A – C, Thyasira anassa sp. nov. Holotype: A – B, external and internal of the left valve; C, gross anatomy viewed from the right side and right mantle removed; D – E, Channelaxinus investigatoris Smith, off Sri Lanka, National Museum of Wales, NMWZ 1955.158.11399; F – H, Thyasira acuticarinata Smith, reproduced from Annandale & Stewart, 1909. aa, anterior adductor muscle; au, auricle; aus auricular sinus; f, foot; id, inner demibranch; lbp, lateral body pouch; lun, lunule; me, mantle edge; od, outer demibranch; pa, posterior adductor muscle; ps, posterior sulcus; psn, posterior sinus; sms, submarginal sulcus; smsn, submarginal sinus.