In LM, valves semi-elliptical with smoothly convex dorsal margin, nearly straight ventral margin and the valve ends narrowly rounded and slightly ventrally curved (Fig. 2A–H). Raphe slightly arched and positioned centrally to slightly ventrally on the valve face. Both dorsal and ventral striae not discernible. Valve length 13.8–15.0 μm; valve breadth 2.8–3.0 μm (n = 31).

In SEM, externally, the central area slightly expanded on the ventral side only (Fig. 2I, K, asterisks). Central raphe endings straight, slightly expanded and positioned very close together (Fig. 2K, arrowhead); distal raphe endings bent towards the dorsal side (Fig. 2L, arrow). Raphe ledge well-developed on the dorsal side of the valve (Fig. 2I, K) and narrows sharply towards the dorsal valve margin. Siliceous outgrowths of various sizes arranged irregularly on the raphe ledge margin (Fig. 2K, arrows). The axial area narrow, and difficult to distinguish because of the fine striae (Fig. 2I).

Dorsal striae slightly radiate. Ventral striae are slightly radiate and more distantly spaced at the valve middle, becoming parallel to slightly convergent and denser near the valve apices (Fig. 2I, K, M). Dorsal striae biseriate under raphe ledge with two rows of small, round areolae under SEM (Fig. 2M, arrow); becoming uniseriate, composed of very small round areolae over the rest of the dorsal side (Fig. 2K, M), 34–38 in 10 μm (n = 13). Ventral striae uniseriate, almost identical in size and shape to dorsal striae, 46–48 in 10 μm (n = 13, Table 1). Axial longitudinal line absent on dorsal side. Internally, longitudinal rib absent. Areolae occluded by hymenes (Fig. 2N, arrows). Proximal raphe endings finish onto small and tongue-shaped fused helictoglossae (Fig. 2N, double asterisk). Poorly developed helictoglossae at distal raphe endings (Fig. 2O, arrow). Round or ovoid poroids in girdle bands, arranged in two rows of 65–67 in 10 μm (Fig. 2P, arrows).

Slide no. MABIK DI00043482 (represented by the valve shown in Fig. 2A) was deposited at the National Marine Biodiversity Institute of Korea (MABIK), located in Seocheon-gun, Chungcheongnam-do, South Korea.

SEM stub no. MABIK DI00043483 and cleaned material no. MABIK DI00043484 (preserved in 99% ethanol).

The intertidal mudflat located in Hampyeong Bay (35°01.89'N, 126°24.31'E), Muan-gun, Jeollanam-do, South Korea (site: HP1-3), July 19, 2018.

The specific epithet “hampyeongensis” refers to the type locality, Hampyeong Bay, Muan-gun, Jeollanam-do, South Korea.

Halamphorahampyeongensis is a benthic species currently known only from the type locality. The sediment temperature and salinity of pore water in the sediment at the time of sampling were measured to be 27.8 °C and 24.9 psu, respectively.

The nucleotide sequences were deposited in GenBank under accession numbers OQ642108 (18S rDNA) and ON137728 (rbcL gene).