Channelaxinus aff. excavata Dall, 1901

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 : 257-258

Channelaxinus aff. excavata Dall, 1901

Figs 3A–B

Afz, anterior flattened zone; lun, lunule; pfz, posterior flattened zone; psn, posterior sinus; sms, submarginal sulcus.

Material. One shell, Off Ras Madrakah, southern Oman, Discovery cruise 211 st. 12719#1, 19°08´N 58°39´E, 3150 m, collected P.G. Oliver, 31 October 1994. NMW.Z. 1995.009.17.

Description. Shell. 11.0 mm in length, 11.5 mm in height. Thin but robust. Equivalve. Equilateral. Outline polygonal, umbos low, beaks prosogyrous; anterior dorsal slope steep, slightly concave; anterior very short almost straight; ventral rounded; posterior ventral short, almost straight; posterior with a deep sinus; posterior dorsal long, straight, sloping steeply. Surface strongly contoured; lunule excavated and bounded by a well developed ridge; anterior slope flattened demarcated by a weak radial ridge; posterior demarcated by a deep sulcus, anterior to it a flattened zone; submarginal sulcus deeply excavated with an elongate auricle that does not rise above the submarginal sulcus. Hinge edentulous, ligament in a shallow depression on the auricle and extending for ~half the length of the auricle. Muscle scars indistinct. Shell chalky white in colour.

Remarks. Shells of this general form are found in the East Pacific ( C. excavata ( Figs 3 C–D) and C. oliveri Valentich-Scott & Coan, 2012 in Coan & Valentich-Scott 2012); Atlantic ( C. perplicata Salas, 1996 ) ( Fig. 3 G); and South Australia ( Prothyasira adelaideana Iredale, 1930 ( Figs 3 E–F) and P. benthicola Iredale 1930 ( Figs 3 H–I)). Oliver & Frey (2014) tentatively linked Cryptodon investigatoris and Conchocele koyamai Habe, 1981 but although they have deep posterior sulci and excavated lunule they are strongly oblique in outline. Smith (1896) reported on a small shell of C. investigatoris which was not oblique and concluded that there was strong allometric growth, shells becoming proportionately longer with age. Comparing the current specimen with the growth lines on C. investigatoris shells reveals that the oblique form is already developed at this size suggesting that the current shell is not a juvenile of C. investigatoris .

With only a single shell at hand and the close similarities with the above species a conclusion cannot be reached.

Iredale (1930) placed both adelaideana and benthicola in his new genus Prothyasira . The type species of Prothyasira is P. peroniana ( Iredale 1930 but while having a very deep posterior sulcus the lunule is not deeply sunken or demarcated by a strong ridge and there are no flattened anterior and posterior zones. These differences suggest that Prothyasira and Channelaxinus are not congeneric and that adelaideana and benthicola be transferred to Channelaxinus .

Figures

FIGURE 3. A – B, Channelaxinus aff. excavatus, Oman margin; C – D, C. excavatus (Dall), holotype, United States National Museum 107449; E – F, C. adelaideana (Iredale), holotype, Australian Museum, C 32091; G, C. perplicata (Salas) as Cryptodon plicatus reproduced from Verrill & Bush, 1898; H – I, C. benthicola (Iredale), holotype, Australian Museum C 47714.