Diagnosis: The body depth distinguishes Diplotaxodon longimaxilla, 28.9-32.4% of standard length, from D. altus, 35.1-37.8% of standard length, D. limnothrissa, 22.5-27.8% of standard length, D. argenteus, 27.3-27.6% of standard length, and D. greenwoodi, 34.2-36.5% of standard length (Ref. 127500). Diplotaxodon longimaxilla, with horizontal eye diameter 27.1-32.1% of head length, has a smaller eye than D. macrops, horizontal eye diameter 35.2-38.2% of head length; the smaller cheek depth of D. longimaxilla, 15.2-19.4% of head length, distinguishes it from both D. aeneus, 23.2-28.1% of head length, and D. apogon, 23.2-28.0% of head length; it has more lateral-line scales than D. ecclesi, 35-38 vs. 32 (Ref. 127500).

Description: Body laterally compressed; head concave just above eye; jaws elongated and prognathous; anterior teeth on lower jaw visible on closed mouth; caudal fin emarginate (Ref. 127500). Small ctenoid scales, with 35-38 lateral-line scales; two rows of scales between upper and lower lateral lines at anterior end of lower lateral line; one scale row posterior between upper and lower lateral lines; one scale row between upper lateral line and posterior insertion of dorsal fin; cheek with three rows of scales, smaller in upper row anteriorly; pored scales posterior to lateral line 1-3 (Ref. 127500). Five to sven gill rakers on outer arch of epibranchial and 16-21 on outer arch of ceratobranchial, with one additional axial element (Ref. 127500). Teeth unicuspid, in 2-3 series, outer teeth larger than inner; outer tooth row with slight inward curve (Ref. 127500).

Colouration: Dark grey dorsally and silver fading to lighter grey ventrally; dark grey caudal fin; two pale yellow ocelli on anal fin of breeding males (Ref. 127500).