Distinguished by the following uniquely derived autapomorphy: typical densely pigmented rectangle like lateral spot located on the 14th or 15th (very rarely 16th) scale of longitudinal line. Can be readily differentiated from its congeners, except Phalloceros tupinamba and Phalloceros aspilos, by the presence a lateral ramus of the female urogenital papilla; female urogenital papilla left turned; large sickle like hook on the gonopodial appendix located in its inner surface and close to its base; and absence of vertical bars along body sides. Phalloceros leptokeras can be distinguished from Phalloceros aspilos by the presence of lateral spot (vs. spot absent). Phalloceros leptokeras can be distinguished from P. tupinamba by the flattened female urogenital papilla (vs. not-flattened in Phalloceros tupinamba) and by the lateral spot densely pigmented, rectangle like, horizontally covering the length corresponding to two or three scales length (vs. normally pigmented, vertically elongated, and horizontally covering the length corresponding to the length of one scale in Phalloceros tupinamba) (Ref. 76852).

Description: Pectoral fin with 6-7 branched rays; longitudinal series of scales 28-29; transverse series of scales 7 (Ref. 76852).