Lannanaia kokensis Jeratthitikul, Sutcharit & Prasankok, 2024, sp. nov.

Lannanaia kokensis sp. nov. Figs 1, 2, 3 Type material. Holotype. Thailand • Chiang Rai Province, Mueang Chiang Rai District, Rop Wiang Subdistrict, Kok River; 19.9131 ° N, 99.8697 ° E; E. Jeratthitikul and K. Wisittikoson leg.; MUMNH -UNI 0825; shell length 57.2 mm, shell height 40.8 mm, and shell width 14.4 mm. Paratype. Thailand • 1 shell; same collection data as for holotype; MUMNH -UNI 0824. Description. Shell medium-sized (shell length 57.1–57.2 mm, shell height 40.8–43.8 mm, shell width 14.4–14.8 mm; Table 2), thin, rather short and high (H / L ratio = 0.71–0.77), winged, sub-trigonal outline, inequilateral, rather compressed. Anterior margin rounded; posterior margin angulate, somewhat pointed; ventral margin curved. Dorsal margin straight; anterior low and short, gradually elevated to posterior end; posterior end high and winged. Umbo tiny, not prominent, eroded. Posterior ridge wide and obtuse, not prominent; posterior slope with two fine lines running as curved line from umbo to posterior margin. Periostracum thin, greenish to dark brown, the eroded part white to coppery-brown. Shell surface with very fine growth lines. Ligament long, very narrow, dark brown. Hinge without dentition, or with very rudimentary broad pseudocardinal tooth in each valve; posterior end of hinge structure with rather broad V-shaped fossette. Anterior adductor muscle scar very shallow, ovate, contiguous with anterior pedal retractor muscle scar, but separated from anterior protractor scar; posterior adductor muscle scar round, very shallow, almost invisible. Pallial line very faint, continuous. Umbo cavity shallow. Nacre blue-whitish. Exhalant aperture smooth, shorter than inhalant. Inhalant aperture with a row of conical papillae, varying in length. Small epithelial fold divides exhalant and inhalant apertures. Gills elongated and slightly ribbed; outer gills much narrower (about half) than inner gills; anterior margin of inner gills slightly longer than that of outer gills. Labial palps elongate, round at tip. Glochidia unknown. Etymology. The species name “ kokensis ” refers to the type locality, the Kok River in Chiang Rai Province, northern Thailand. Distribution. This new species is known only from the type locality. Remarks. Two living specimens were collected from a small stream that is a tributary to the Kok River, with a muddy bottom. We searched multiple times and locations for this species but failed to find additional specimens. In contrast, Lens contradens (Lea, 1838) and Pilsbryoconcha exilis (Lea, 1838) were found nearby in high abundance, both living and as empty shells. We assume this new species has a low population density and is possibly endemic to the Kok River Basin.