Gymnostomus

Following Roberts (1997) G. ariza can be diagnosed by the following combination of characters: body with variably intense thin stripes dustributed mostly above the lateral line; larger individuals sometimes with a broad midlateral stripe; 32-35 lateral scales; 7-8/1/5-6 transverse scale rows; 8-9 branched dorsal-fin rays 8-9; 22-24+ 11-12=34(4), 35 (3) vertebrae; live colour pattern variable, overall dull dirty white to greyish, silvery or yellow.

Recorded from the middle to lower Mekong river basin in southern China (Yiunnan province), Laos, Thailand, and Cambodia, plus the Chao Phraya watershed in central Thailand. Type locality is 'Thailand: Lam Ton Lang, a tributary of Menam Sak [Mae Nam Pa Sak; Ban Lam Thong Lang, village northwest of Pakjong'.

G. lobatus can be distinguished from congeners by the following combination of characters: possession of 32-33 + 2-3 lateral line scales; 5½ scale rows between lateral line and base of dorsal-fin; body relatively slender, depth fits 3.7-4.0 times in SL; snout often projecting strongly; body plain silver in colour.

G. ornatapinnis can be distinguished from congeners by the following combination of characters: head small; snout relatively short and blunt; caudal peduncle very deep; pectoral, pelvic, and anal fins usually reddish-orange or rose-tinted; pelvic and anal fins often with white tips; barbels completely absent.

G. siamensis can be distinguished from congeners by the following combination of characters: possession of 5½ lateral line scales; body relatively deep, depth fits 3.2-3.4 times in SL; head large and broad, width fits 5.5-6.7 times in SL; snout not or only weakly projecting; body plain silver in colour with no dark marking on caudal peduncle.