Morphology of the postotic laterosensory canal was surveyed across loricarioid and outgroup catfishes in order to resolve conflicting statements regarding homology and phylogenetic significance of intrinsic character variation. A pterotic branch is widespread among catfishes and has been identified as a synapomorphy for siluriforms, but its presence in loricarioid catfishes has been disputed. In contrast to previous statements that absence of a pterotic branch is synapomorphic for loricarioids, we confirm the presence of a pterotic branch in Nematogenys inermis and other trichomycterids, callichthyids, and loricariids. The pterotic branch is secondarily absent in scoloplacids and astroblepids. We present criteria for establishing homology of the pterotic branch and review character state optimization schemes on the currently accepted phylogeny. The postotic region of loricariids is further specialized in having an expanded swimbladder capsule that incorporates the trunk lateral line canal and has a lateral opening covered by a greatly expanded pterotic complex. The trunk lateral line enters the swimbladder capsule mesial to the pterotic lateral wall and passes anteromedially as a fleshy tube before forming the postotic canal in the pterotic, a morphology reported previously for a single loricariid representative. Variation in the relative extent and topographic position of postotic canal branches and other morphologies is diagnostic of certain loricariid taxa, suggesting a rich character complex of potential utility in phylogeny reconstruction. J. Morphol. 246:212–227, 2000. © 2000 Wiley‐Liss, Inc.

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A new species of the hypoptopomatine genus Hisonotus (Loricariidae) is described from a small tributary of the upper río Uruguay basin near the border between Uruguay and Brazil. The new species can be distinguished from all other congeners by the following combination of characters: (1) presence of serrae along distal two thirds of posterior margin of pectoral-fin spine (versus serrae absent, posterior margin smooth); (2) odontodes along anterior margin of snout biserially arranged, dorsad and ventrad series separated by narrow odontode-free area covered by pad of soft tissue; (3) caudal peduncle short (27-34% SL, versus Ͼ 34% SL) and deep (13-15 % SL, versus Ͻ 13% SL); (4) eye large (15-19% HL, versus Ͻ 13% HL); and (5) caudal-fin pigmentation, when well defined, dark brown with a pair of whitish blotches on upper and lower lobes. The significance of the distribution of the new species is discussed relative to the degree of endemism of other fish groups in the Uruguay basin. RESUMENUna nueva especie de Hypoptopomatinae del género Hisonotus (Loricariidae) es descripta para un pequeño tributario del río Uruguay superior, cerca del límite entre Uruguay y Brasil. La nueva especie puede distinguirse de todas las otras especies nominales del género por la siguiente combinación de caracteres: (1) presencia de sierra a lo largo del margen posterior de los dos tercios distales de la espina pectoral (versus margen posterior liso), (2) odontodes del margen anterior del hocico ordenados biserialmente, las series dorsal y ventral separadas por una banda angosta libre de odontodes, cubierta por tejido blando; (3) pedúnculo caudal 2 NO. 3333 AMERICAN MUSEUM NOVITATES corto (27-34 % LE, versus usualmente Ͼ 34) y alto (13-15 % LE, versus usualmente Ͻ 13); (4) ojo grande (15-19 % in HL, versus usualmente Ͻ 13), y (5) patrón de coloración de la aleta caudal, cuando se encuentra bien definido, marrón oscuro, con un par de manchas blanquecinas sobre los lóbulos superior e inferior de la aleta. La distribución geográfica de la nueva especie es discutida en relación al grado de endemismo registrado en otros grupos de peces de la cuenca del río Uruguay.

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