Rugalucina vietnamica (Zorina, 1978). A–C Pillucina vietnamica Zorina, 1978 syntypes (ZISP), exterior of right valve and interior of left and right valves, Hainan, China, L 5.5–8.9 mm D internal drawing of C. E, F R. vietnamica Palau Semaku, Singapore (NHMUK 20150057), L 8.5 mm G–L R. vietnamica Kungkraben Bay, eastern Thailand (NHMUK 20191072) G exterior of right valve, L 7.0 mm. H exterior of left valve, L 7.0 mm I, J interior of left and right valves, L 7.7 mm K dorsal view L 5.6 mm L, M detail of hinge of I and J. N Section through gill of Rugalucina vietnamica Kungkraben Bay, Thailand showing filaments with ciliary (cz) and bacteriocyte zones (bz). Critical-point dried preparation O detail showing abundant bacterial symbionts in bacteriocytes. Scale bars: 20 µm (N); 10 µm (O).