| Summary: | Acesta cryptadelphe sp. nov. Figs. 2, 3 C–D, 6, 14–16 Material examined. Collection data for each sample set is shown in Table 2. The holotype and three paratypes are from The Gully, off Nova Scotia; nine paratypes are from Beothuk Knoll, off northeast Newfoundland (southwest Flemish Cap), including the tissue from two specimens separated from their respective shell; and two paratypes are from the main basin of Bay d’Espoir, southern Newfoundland (Gagnon & Haedrich 2003). Table 3 summarizes the catalogue/accession information and basic morphological data for each specimen included in the type series. All specimens are deposited in the Mollusc Collection of the Canadian Museum of Nature; five of these were used to extract partial COI gene sequence datasets, which are available on GenBank. Description. Holotype (CMNML 097156): Complete specimen, left valve preserved dry (Fig. 14 A), right valve (posterior margin damaged) and tissue preserved in 70% ethanol; nearly equivalved (see byssal notch below), thin, ovate with narrow hinge region (hinge length to shell width ratio = 0.43; Tab. 3), exterior light beige in colour, with whitish radial ribs in median portion of shell; anterior and posterior radial ribs most prominent, attenuating toward median region, becoming broader and flattened; at least 8–9 major growth lines irregularly interrupting radial ribs. Poorly developed anterior auricle, dorsally delineating lunule; posterior auricle extending from umbo and representing about 83% of hinge length (Tab. 3). Lunule length 3.5 cm, representing 41% of shell length, with weak inward curve. Byssal notch indistinct on left valve with lunule inner margin nearly straight, slightly gapping on right valve, particularly toward the dorsal region of the lunule. Shell moderately inflated with highest inflation point at first third of shell length. Shell interior translucent, glossy, whitish, with weak but demarcated muscle scars in the dorsoposterior region of the shell; pallial line weak. Hinge plate with deep, oblique, ...
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