Diagnosis: Trematocranus pachychilus is clearly distinct from all other known species of the genus by its thick lips (Ref. 120598). It can further be distinguished from T. labifer by its molariform pharyngeal dentition while the latter has slender teeth on the lower pharyngeal jaw (Ref. 120598). In T. placodon lips are usually very small; while T. pachychilus has a rather concave head, as in T. microstoma, T. placodon has a more distinct convex head; in both T. pachychilus and T. placodon, the smaller specimens have a straighter head profile; coincidentally, in two large specimens of T. placodon the head is slightly concave; the teeth of T. pachychilus are less broad, more densely spaced and slightly less numerous, 36-47 vs. 41-57, than in T. placodon; Trematocranus pachychilus has more upper lateral line scales, 22-24 vs. 19-22, and more dorsal fin rays, 11-12 vs. 9-11, than T. placodon; the pharyngeal bones of T. pachychilus are less developed than in T. placodon; they are shallower, 9.4-10.4% of head length vs. 13.8-16.8%, narrower, 29.7-33.2% of head length vs. 36.1-48.7%, and shorter, 21.8-30.8% of head length vs. 29.8-35.9%, than those of T. placodon (Ref. 120598). The dentigerous area is also narrower than in T. placodon, 21.9-24.4% of head length vs. 24.2-27.5%; both species have large, molariform pharyngeal teeth, although in T. pachychilus the lateral teeth are smaller and more numerous than in T. placodon; the number of teeth on the posterior pharyngeal row in T. pachychilus is higher than in T. placodon, 23-27 vs. 11-23 (Ref. 120598). In T. microstoma the lips are intermediate; the snout of T. pachychilus is shorter than that of T. microstoma, 34.8-40.9% of head length vs. 38.0-42.4%; Trematocranus pachychilus has bicuspid oral teeth on the outer rows while T. microstoma has slender unicuspid teeth; it has fewer inner teeth rows, 3-5 vs. 6-8 on the lower jaw than T. microstoma; the lachrymal depth is clearly shorter than in T. microstoma, 22.9-29.7% of head length vs. 29.3-32.0%, while the eye diameter is larger, 25.2-29.5% of head length vs. 21.2-25.5%; the pharyngeal bones are more developed in T. pachychilus than in T. microstoma; the dentigerous area of T. pachychilus is wider than in T. microstoma, 21.9-24.4% of head length vs. 18.8-23.6%; the dentition of the lower pharyngeal bone is similar in both species, with the median teeth enlarged and the lateral teeth small and numerous (Ref. 120598).

Description: Body deep and laterally compressed; head steep; most specimens with a slight concavity at eye level; some, especially smaller specimens, with a merely straight head profile; snout pointed; mouth terminal; lips very thick and equally developed in most specimens; larger specimens with thicker lips than small specimens; smaller specimens often with a clear boundary between lower lip and chin; maxilla does not extend to level of eye (Ref. 120598). Teeth not readily observable, being to a large extent or fully covered by the fleshy gums; outer row of teeth on upper and lower jaws with mainly unequally bicuspid and some unicuspid teeth in the postero-lateral parts; teeth slender, generally straight and slightly curved inwards; crown slightly wider than base; anterior bicuspid teeth more pointed and sharp; lateral bicuspid teeth often rounded; inner rows mostly with unicuspid teeth; larger specimens with more bicuspid teeth anteriorly in inner rows; inner teeth rows difficult to observe; 3-4 inner rows on upper jaw; 3-5 on lower jaw (Ref. 120598). Pectoral fins implanted slightly behind level of dorsal fin origin; pelvic fin origin positioned slightly more backwards; anal fin implanted anterior to level of first soft dorsal fin ray (Ref. 120598). Lower pharyngeal bone triangular, large, wider than long, and deep; teeth molariform; central teeth much larger than lateral teeth; outermost teeth on the posterior rows very small and numerous (Ref. 120598).

Colouration: Colour pattern in preservation: body generally brown; some specimens slightly more greyish; dorsum darker than belly; very dark-coloured on the dorsal parts of head and body contiguous with dorsal fin base in most specimens; some large darker spots usually on operculum, supra-pectoral in front of dorsal fin origin, dorso-lateral above and on upper lateral line in middle of flank, supra-anal near end of upper lateral line, and caudal peduncle; clear dark maculae on spiny part of dorsal fin; sometimes also on soft dorsal fin part and caudal fin; pelvic and anal fins yellowish brown in females; fin base and distal part clearly darker in males (Ref. 120598).