Diagnosis: Cryptomyrus ona is readily differentiated from its sole congener, C. ogoouensis, by the following characteristics: having an anal-fin origin located only just in advance of the dorsal fin, first dorsal ray above anal-fin ray 3 vs. first dorsal-fin ray above anal-fin ray 7; a deep caudal peduncle, 6.0-6.8% of standard length vs. 5.1%; and lobes of caudal fin markedly shorter than caudal peduncle, vs. nearly as long as peduncle (Ref. 113668).

Description: Body fusiform with dorsal and ventral profiles gently convex, greatest body depth between terminus of pelvic fin and urogenital pore; body compressed, widest at head; dorsal head profile gently convex with shallow slope downwards to snout, slight inflection between snout and head above eye; ventral profile of head with marked concavity between gular region and chin, with body depth increasing rapidly from here to below pelvic fin origin (Ref. 113668). Snout bulbous, rounded, tip below horizontal through center of eye, projecting above upper lip; nostrils well separated from each other and from eye; posterior naris at level of bottom of orbit, anterior naris below line through center of eye; straight line drawn through nostrils passes through tip of snout and just below pectoral-fin origin (Ref. 113668). Mouth subinferior, small; rictus below posterior naris, in advance of eye; chin swelling modest, but broad, nonprotrusive (Ref. 113668). Premaxillary teeth 5, strongly notched, dentary teeth 6, spatulate, center four oriented horizontally, center two with broad contact between inner edges which are longer than outer edges, projecting well beyond neighboring pair to which they are closely appressed, trowel-like (Ref. 113668). Eye large, laterally positioned on head (Ref. 113668). Predorsal distance equal to preanal distance; anal fin only slightly in advance of dorsal fin; first dorsal-fin ray above first or second branched anal-fin ray; dorsal and anal fins with short bases; dorsal-fin rays 3+18, anal-fin rays 3+21; 2 anal-fin rays beyond last dorsal-fin ray; dorsal fin with falcate posterior margin; longest dorsal-fin ray is first branched ray; next 10 rays successively shorter; next 5 rays approximately equal in length, final 2 rays longer; anal-fin ray with gently falcate posterior margin; longest anal-fin ray is second branched ray; next 10 rays successively shorter, next 6 rays more or less equal in length, final 3 successively longer (Ref. 113668). Pectoral fin with 10 rays, tip pointed, extending beyond the origin of the pelvic fin, but short of halfway; pelvic fin with 6 rays, closer to pectoral than to anal (Ref. 113668). Caudal peduncle deep, depth at middle of peduncle slightly deeper than at origin; caudal fin deeply forked with 20 rays in each lobe, lobes short, scaled at their bases with bluntly pointed ends; distance from caudal flexion to caudal tips shorter than caudal peduncle length (Ref. 113668). Body scales large along sides, smaller dorsally; 42 pierced lateral line scales and 2 unpierced along midlateral line, 18 sclaes in transverse series between origins of dorsal and anal fins, 9 from pelvic fin to midlateral line, 12 around caudal peduncle (Ref. 113668). Total vertebrae 41, epineurals associated with vertebrae 1-8, pleural ribs directly attached to vertebral centra 2-13, ribs displaced beneath haemal arches on vertebrae 14-17, caudal vertebrae 18-41; hypurals 1 & 2 unfused (Ref. 113668).

Colouration: In life, a purplish-gray body, darker along dorsum, with whitish marbling/speckling on lower half of head and along belly to anal fin and diffuse band of pigment below dorsal fin occupying 4 scales, darker above, lighter or absent above anal fin; numerous depigmented spots over electroreceptors conspicuous on snout, top of head, belly and upper back; fins with darkly pigmented rays, interradial membranes hyaline (Ref. 113668). In preservation, body yellowish tan (Ref. 113668).