Petrocephalus binotatus, described by Pellegrin (1924), is a small sized species within the genus Petrocephalus (maximum standard length observed = 88.4 mm, holotype = 83.2 mm). Body ovoid, body 2.3 to 2.8 longer than high (holotype = 2.4) and laterally compressed. Head length between 3.6 and 4.0 times in standard length (average = 3.9, holotype = 3.6). Snout short and round. Eye large (3.5 ≤ Head length/eye diameter ≤ 4.0, average = 3.6, holotype = 3.6). Mouth small (4.4 ≤ Head length/head width ≤ 5.2, average = 4.7, holotype = 3.2), sub–terminal, opening under the anterior half of the eye. Dorsal fin shorter than the anal fin (anal fin length/dorsal fin length ≥ 1.5, range = 1.5–1.7). Dorsal fin with 20-24 branched rays. Anal fin with 30-33 branched rays. Teeth small and bicuspid, 16 teeth or fewer (range = 8–16, median = 10) in the upper jaw, 24 teeth or fewer (range = 19–24, median = 21) in the lower jaw. Mouth sub–terminal; ratio of head length to mouth position between 3.9 and 6.1. Scales cover the body, except for the head. Lateral line visible and complete with 37 to 38 pored scales along its length. Eleven to 13 scales (median = 12), between the anterior base of the anal fin and the lateral line. Caudal peduncle thin (1.8 ≤ Caudal peduncle length/Caudal peduncle depth ≤ 2.2, average = 2.0, holotype = 2.3). Twelve scales around the caudal peduncle. Skin on head thick, becoming opaque with formalin fixation. Three distinct rosettes of Knollenorgans are present on the head (Augenrosette, Nackenrosette and Kehlrosette).
Body uniformly white-silver, with the presence of three characteristic melanin marks on each side of the body: (1) a distinct, approximately round/oval black mark situated slightly anterior to the dorsal fin, sometimes covering only a few scales; (2) a black spot at the base of the pectoral fin; (3) an ovoid black mark centered at the base of the caudal fin that does not extend onto the upper and lower parts of the caudal fin. Fins translucent.
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