Ascetoaxinus ovoidea ( Dall, 1890 )

Oliver, P. Graham & Frey, Melissa A., 2014, Ascetoaxinus quatsinoensis sp. et gen. nov. (Bivalvia: Thyasiroidea) from Vancouver Island, with notes on Conchocele Gabb, 1866, and Channelaxinus Valentich-Scott & Coan, 2012, Zootaxa 3869 (4), pp. 452-468 : 457

Ascetoaxinus ovoidea ( Dall, 1890)

Fig. 7

Type material. Holotype, single specimen, 87 miles off Cape Fear , North Carolina, United States, 646m. Coll. United States Fish Commission. USNM 64226 .

Synonymy. Cryptodon ovoideus, Dall, 1890 ; Thyasira ovoidea ( Dall, 1890) .

Measurements.

Description. Shell ( Fig. 7 ). 25mm in length. Brittle, chalky. Equivalve. Moderately tumid. Strongly inequilateral, prosogyrous beaks close to the anterior margin. Outline obliquely oval; anterior margin almost straight bounding a large excavated lunule; ventral margin long, almost straight; posterior margin broad, sulcate with a distinct posterior dorsal sinus and faint submarginal sulcus. Posterior sulcus sharply defined, relatively narrow; submarginal sulcus sharply defining a projecting escutcheon. Posterior ventral slope anterior of the posterior sulcus a little flattened, creating a weak secondary ridge. Hinge teeth lacking; ligament partially sunken, relatively short with length less than half that of escutcheon. Sculpture of well-defined growth lines; edge of lunule drawn out into two rounded projections. Muscle scars prominent; anterior adductor scar elongate, mostly separate from pallial line, and in parallel with ventral margin. Shell colour, creamy-white.

Remarks. Based on putative geographic distributions coupled with obvious biogeographic barriers, it is most unlikely that Ascetoaxinus quatsinoensis is conspecific with A. ovoidea . Panamic and Caribbean geminate taxa in the Arcoidea have been shown to be distinct species ( Marko and Moran, 2009) and this probably applies to most bivalves. Shell morphology differences are present; in A. quatsinoensis the scalloping of the lunule is more prominent as is the definition of the escutcheon.

Figures

FIGURE 7. Ascetoaxinus ovoidea (Dall, 1890), Holotype, USNM 64226. A, external of left valve; B, oblique dorsal view showing posterior and submarginal sulci; C, internal of left valve; D, oblique view of the lunule. lig, ligament; lun, lunule; ps, posterior sulcus; sms, submarginal sulcus.