Valve oblong with broadly rounded poles, narrow elliptical to elliptical, 6.7–16.6 μm long and 2.0–3.7 μm wide. Raphe filiform and slightly curved, central endings straight and distant externally, internally deflected in the same direction and relatively closed to each other. Polar raphe terminal fissures curved to the same side externally and ended by helictoglossae internally. Striae radiate, 41–46 in 10 μm, restricted at valve ends. Striae composed of two elongate areolae occluded by hymenes, the proximal one internally occluded, the distal one elevated and occluded externally. Conopeum without perforations, covering about 1/2 of valve width. Transapical microcostae inconspicuous.

Type:— JAPAN. Miyajima Island: Hiroshima Prefecture, Hatsukaichi City, 34° 14’ 28.71” N, 132° 16’ 16.07” E, Yuhang Li, 4 November 2012 (holotype BM!101827, here illustrated in Fig. 4, The Natural History Museum London, UK; isotype MBM! 283387, Marine Biological Museum, Chinese Academy of Sciences, Qingdao, China).

Etymology:—The Latin adjective salinus (saline) refers to the habitat of the species, which was found from brackish to saline water.

Observations:—Valves are oblong with broadly round poles, or narrowly elliptical to elliptical, 6.7–16.6 (9.4 ± 2.3) μm long and 2.0–3.7 (2.9 ± 0.4) μm wide (Figs 2–16). Raphe is filiform and slightly curved (Figs 17, 18). Raphe terminal fissures extend to the valve mantle and are curved to the same direction externally (Fig. 17), whereas raphe distal endings form small helictoglossae internally (Fig. 24, arrow). The external central raphe endings are straight and distant (Figs 20, 22, arrows). The internal central raphe endings (Figs 21, 22, arrowheads) deflect to the same direction and are relatively closed comparing with external ones. The distance between two central raphe endings takes about 1/4 of valve length, whereas 1/5 of valve length internally (Figs 17, 18). Striae are radiate and only present at both valve ends, 41–46 (43 ± 2) in 10 μm (Figs 18, 19, 25). Each stria is composed of two elongate areolae (Figs 23, 25, arrows and double arrows) occluded by hymens (Fig 28, arrows). The proximal areola (Fig. 28) is occluded internally (Figs 23, 27, arrows), whereas the distal one is occluded externally on the thickened and elevated valve margin (Figs 23, 27, double arrows). Conopea are nonperforated and extended from the thickened raphe sternum, taking about 1/2 of valve width (Figs 17, 19, 26, arrow). The transapical costae are inconspicuous at plan view but could be observed clearly at a tilt view (Fig. 23, arrowheads). The cingulum consists of two bands at least, including one valvocopula (Fig. 26, arrowhead).

Distribution and ecology:— Microcostatus salinus has been found at the mouths of two small rivers in the Miyajima Island, Hatsukaichi City, Hiroshima Prefecture and in the Ena Bay, Miura Peninsula, Kanagawa Prefecture, Japan. This species inhabits sandy sediments in brackish to saline water.