Routine examination of gas bladders uncovered a new species of Hassar from the upper Tapajós basin, Mato Grosso, Brazil The new species, Hassar shewellkeimi, is distinguished from all congeners by its relatively long snout and narrow interorbital, distance from snout to anterior naris 2.2–3.1 times interorbital width (vs. 1.1–2.2 in congeners). Hassar shewellkeimi most closely resembles H. orestis, but is further distinguished from that species by having gas bladder with few, weakly developed peripheral diverticula restricted to anterolateral shoulder and posterolateral face of anterior chamber, and sometimes anterolateral face of posterior chamber (vs. diverticula well developed, with multiple branches and grouped into fascicles encircling horizontal periphery of gas bladder in H. orestis), and posterior face of gas bladder smooth, broadly to narrowly rounded (vs. posterior chambers expanded posteriorly into medially conjoined terminal diverticula, effecting acutely subtriangular, cone-shaped posterior face). Characteristics for distinguishing the five valid species of Hassar are summarized, and their known ranges are expanded. In addition, all four specimens in the original syntype series of Hassar wilderi Kindle 1895 are located, and dueling lectotype designations are clarified with CAS 60711 [ex CU 1703 or 1705, ex IUM 5120; 139.5 mm SL, 167 mm TL] recognized as the valid lectotype established by Eigenmann (1925:364, Plate XXII, fig. 2).

New species: Hassar shewellkeimi Sabaj Perez & Birindelli