Deep-water Thyasiridae (Mollusca: Bivalvia) from the Oman Margin, Arabian Sea, new species and examples of endemism and cosmopolitanism Author Oliver, P. Graham text Zootaxa 2015 3995 1 252 263 journal article 10.11646/zootaxa.3995.1.21 810c1097-eff4-4262-acd6-1f76d05f1f83 1175-5326 236857 1D5CD052-9754-493C-836D-6A69756747D9 “ 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.