Petrocephalus mbossou is a large–sized species within the genus Petrocephalus (standard length = 127.1 mm). Body ovoid, longer than high (standard length /height = 3.0) and laterally compressed. Head length 3.4 times in standard length. Eye large (Head length/eye diameter = 3.7). Snout short and round. Mouth small (Head length/mouth width = 4.7), opening ventrally under the posterior half of the eye. Teeth small and bicuspid, 12 in a single row in the upper jaw, 14 in a single row in the lower jaw. Both the dorsal and anal fins originate in the posterior half of the body (standard length/pre–dorsal distance = 1.7 and standard length/pre–anal distance = 1.6, respectively). The pre–dorsal distance is slightly greater than the pre–anal distance. Dorsal fin with 24 branched rays. Anal fin with 26 branched rays. Scales cover the body, except for the head. Lateral line visible and complete with 37 pored scales along its length. Caudal peduncle relatively thin (caudal peduncle length/caudal peduncle diameter = 2.9) with twelve circumpeduncular scales. Skin on head thick, turning opaque with formalin fixation and containing numerous Knollenorgan electroreceptors organized into only two rosettes (the Nackenrosette and the Kehlrosette). The Augenrosette appears to be absent.
Body mostly white–silver. Dorsum slightly darker than the rest of the body. Pigmentation pattern consists of two distinct black markings: (1) a black patch of irregular shape below the anterior base of the dorsal fin (first ray to the sixth or seventh ray); (2) an irregularly-shaped mark centered at the base of the caudal fin that does not extend onto the rayed portions of the upper and lower caudal fin lobes. The fins themselves are translucent.
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