Petrocephalus balayi, described by Sauvage (1883), is a large, robust species within the genus Petrocephalus (maximum standard length = 95.6 mm SL). Body ovoid, body 2.5–2.8 longer than high and laterally compressed. Head length between 3.3 and 3.4 times (holotype = 3.3) in standard length. Head width 1.9 times (holotype = 2.2) in head length. Snout short, wide and square–shaped. Mouth wide (2.7 ≤ Head length/Mouth width ≤ 3.1, holotype = 3.4), sub–terminal, opening under the anterior half of the eye. Teeth small and bicuspid, 30 in a single row in the lower jaw and 20–21 in the upper jaw. Eye small (4.5 ≤ Head length/Eye diameter ≤ 4.8, holotype = 4.6). Dorsal and anal fins originate in the posterior half of the body (standard length/pre–dorsal distance = 1.6 and standard length/pre–anal distance = 1.6), with pre–dorsal distance equal to pre–anal distance. Pre–dorsal distance slightly exceeds pre–anal distance in the holotype. Dorsal fin with 22 branched rays (holotype = 21). Anal fin with 27 branched rays (holotype = 26). Scales cover the entire body, except for the head. Lateral line visible and complete with 36 pored scales. Caudal peduncle relatively thick (1.7 ≤ CPL/CPD ≤ 1.8, holotype = 2.3). Twelve circumpeduncular scales. Skin on head thick, becoming opaque with formalin fixation. Knollenorgans organized into the three rosettes.
Body gray/silver, slightly darker dorsally. The head is also slightly darker than the rest of the body. Iridescent pigment along side of body sometimes visible with correct orientation of light. Pigmentation pattern with three black patches: (1) a distinct round black mark on each side of the body below the dorsal fin origin; (2) an ovoid black mark on each side at the base of the caudal peduncle that does not extend onto the upper and lower parts of the caudal fin; (3) a black mark, sometimes diffuse in larger individuals but always present, at the base of the pectoral fins. The fins themselves are translucent.
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