Diagnosis: Amphilius crassus is diagnosed from A. jacksonii, A. ruziziensis, A. pedunculus and A. lujani by its shorter caudal peduncle, caudal peduncle length 13.3-15.5% of standard length vs. 16.0-20.9%, and from A. jacksonii, A. ruziziensis and A. lujani by its deeper caudal peduncle , caudal peduncle depth 10.1-12.0% of standard length vs. 4.8-9.5% (Ref. 103388). It is diagnosed from A. ruziziensis, A. pedunculus and A. frieli by its longer dorsal-fin insertion to adipose-fin insertion length, 42.2-44.6% of standard length vs. 37.5-41.7% (Ref. 103388). Amphilius crassus is further diagnosed from A. jacksonii by its deeper body, body depth at anus 14.5-17.4% of standard length vs. 9.6-13.2%, and from A. ruziziensis by wider interorbital width, 28.9-34.1% of head length vs. 23.4-25.1%, and deeper body, body depth at anus 14.5-17.4% of standard length vs. 11.1-12.9%; it is further diagnosed from A. lujani by having fewer branched pectoral-fin rays, 7-8, rarely 9 vs. 9, and from A. frieli by its fewer branchiostegal rays, 6-7, rarely 8 vs. 8-9, fewer branched pectoral-fin rays, 7-8, rarely 9 vs. 9-10, fewer gill rakers on the first gill arch, 6-8, rarely 9 vs. 10-11, rarely 9 or 12, and wider interorbital width, 28.9-34.1% of head length vs. 25.1-27.7% (Ref. 103388).

Description: Body elongate; ventral profile flattened ventrally to anal-fin base, then tapered dorsally to end of caudal peduncle; dorsal profile rising steeply from tip of snout to dorsal-fin origin, then nearly horizontal to end of caudal peduncle; greatest body depth at dorsal-fin origin (Ref. 103388). Caudal peduncle laterally compressed, with crenellated epidermal fold; anus and urogenital openings located at midpoint of adpressed pelvic fin, closer to pelvic-fin insertion than to origin of anal fin (Ref. 103388). Skin smooth; lateral line complete, extending from dorsal edge of opercular cavity to caudal-fin base (Ref. 103388). Head and anterior part of body depressed and broad; head wedge-shaped in lateral view; snout broad, blunt when viewed from above; head becoming wider from tip of snout to pectoral-fin base; branchiostegal membranes moderately joined at isthmus forming a V-shaped connection (Ref. 103388). Mouth broad, gently curved, subterminal; lips moderately fleshy, slightly papillate; rictal lobe large and slightly papillate; anterior portion of premaxillary tooth band exposed with mouth closed; premaxillary tooth patches joined, forming crescent shaped band; premaxillary and dentary teeth short, conical; dentary tooth patches forming U-shaped band, separated medially (Ref. 103388). Three pairs of simple, tapered circumoral barbels; maxillary barbel large, fleshy and flattened with pointed tip; barbel extending posterolaterally from corner of mouth, to pectoral-fin base; outer mandibular barbel thin with pointed tip, origin at posterior corner of lower jaw, extending to origin of pectoral-fin; inner mandibular barbel originates anterolaterally of outer mandibular barbel, extending to edge of branchiostegal membrane (Ref. 103388). Branchiostegal membrane with 6-8 rays; gill rakers on first epibranchial 1-3; rakers on first ceratobranchial 4-6; total gill rakers on first arch 6-9 (Ref. 103388). Eye small, positioned dorsolaterally approximately midway between tip of snout and posterior margin of operculum; horizontal diameter of eye slightly wider than vertical diameter; eye without free orbit; covered with skin confluent with dorsal surface of head; anterior and posterior nares with prominent tubular rims; nares separate but relatively close to each other; posterior nare located about midway between eye and tip of snout (Ref. 103388). Dorsal-fin origin at point over tip of pectoral fin; dorsal fin with i,6 rays, and fin margin straight; pectoral fin with i,7-9 rays; unbranched ray greatly thickened; pectoral fin with four or five innermost rays progressively shorter making posterior fin margin rounded; origin of pelvic fin posterior of dorsal-fin insertion; pelvic fin with i,5 rays with first ray unbranched and greatly thickened; pelvic fin with straight posterior margin (Ref. 103388). Adipose-fin base longer than anal-fin base, origin anterior to origin of anal-fin base, fin extending past anal-fin insertion; margin strongly convex with sharply rounded edge, deeply incised posteriorly; caudal fin deeply forked with tips of lobes rounded; fin with i,5-6,5-6,i principal rays; anal fin with short base, origin posterior to origin of adipose-fin base, with ii-iii,5-7 rays; anal-fin margin almost straight (Ref. 103388).

Colouration: Body variably mottled with dark saddles; first saddle posterior of head, second saddle at dorsal fin, third saddle between dorsal and adipose fins, fourth saddle under anterior part of adipose fin, and fifth saddle on caudal peduncle; all saddles connected laterally by broad stripe; venter light brown with fourth and fifth saddles meeting those of opposite side (Ref. 103388). Dorsal and anal fins light brown with dark medial band; adipose fin dark brown to black, with anterior and posterior distal edges cream-coloured; pectoral and pelvic fins positioned horizontally with upper surfaces cream-coloured with dark fin base and dark medial band; lower surfaces light yellow; caudal fin cream-coloured with dark markings on upper and lower lobes; caudal colouration variable but usually spotted and asymmetrical, with lower lobe more pigmented (Ref. 103388).