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Recent study of the type-material of Tembeassu marauna, a poorly-known species currently represented in collections only by its holotype (male) and paratypes (male and female), all collected forty years ago at the Ilha Solteira reservoir area, upper Paraná river region, revealed an unique pattern of dental arrangement among members of the order Gymnotiformes. This autapomorphic condition refers to a patch of around 15 elongate conical, extra teeth associated to soft tissue, and disposed inside the mouth on a restricted area at the roof of oral cavity and located just in front of the premaxillary bones (although clearly apart from those). Besides this obvious feature, two additional conditions are recognized as possibly unique for T. marauna, which refer to the conspicuous "fleshy" (that is, not presenting subjacent bony structures) anterior elongations of the upper and lower jaws. Some additional osteological features identified on radiographs taken from the type material belonging to that species are presented. A brief discussion is furnished, indicating that the recent inclusion of the nominal Tembeassu marauna in Apteronotus La Cépède is not supported by currently available evidence. Thus, it is herein suggested that the species is kept in it own separate genus within the Apteronotidae. Finally, it is argued that T. marauna possibly represents an endangered species at the upper Paraná river region.Estudo recente do material-tipo de Tembeassu marauna Triques, espécie pouco conhecida e atualmente representada em coleções apenas por seu holótipo (macho) e dois parátipos (macho e fêmea), todos coletados há quarenta anos na área do reservatório de Ilha Solteira, região do alto rio Paraná, revelou um padrão de dentição único dentre membros da ordem Gymnotiformes. Esta condição autapomórfica refere-se a um conjunto de cerca de 15 dentes extras, cônicos e alongados, associados ao tecido mole e dispostos no interior da boca em uma área restrita do teto da cavidade oral, posicionados logo à frente dos ossos premaxilares, porém claramente dissociados dos mesmos. Além desta característica óbvia, são também reconhecidos como caracteres supostamente únicos de T. marauna os conspícuos prolongamentos carnosos (isto é, que não são suportados por estruturas ósseas subjacentes) das extremidades anteriores da maxila e da mandíbula. Algumas outras características osteológicas identificadas em radiografias do material tipo da espécie são apresentadas. Uma discussão breve é fornecida, indicando que a inclusão da espécies nominal Tembeassu marauna em Apteronotus La Cépède não se sustenta em evidências atualmente disponíveis. Assim, sugere-se que esta espécie deve ser mantida em seu próprio gênero separado dentro de Apteronotidae. Finalmente, são apresentados argumentos no sentido de reconhecer T. marauna como uma espécie possivelmente ameaçada de extinção na região alto rio Paraná.

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Eigenmannia correntes, a new species belonging to the Eigenmannia trilineata species-group, is described from tributaries of rio Correntes, a major affluent of the rio Piquiri system, upper rio Paraguai basin (Mato Grosso and Mato Grosso do Sul states, Brazil). Eigenmannia correntes is included in the currently poorly defined sternopygid genus Eigenmannia (Ostariophysi: Gymnotiformes) by presenting characters that are either primitive or of uncertain polarity, such as eyes covered by skin, scales present over entire postcranial portion of body, teeth absent from oral valve, infraorbital bones 1+2 with enlarged posterodorsal expansion, and gill rakers short and unossified. The new species is distinguished from all congeners, except those species included in the Eigenmannia trilineata species-group, by the presence of a conspicuous superior midlateral stripe (synapomorphy of this clade). Eigenmannia correntes can be differentiated from all members of the Eigenmannia trilineata species-group, except E. vicentespelaea, E. waiwai and E. besouro, by its subterminal mouth (vs. terminal in remaining species of that group). Further, it differs from these aforementioned species by a number of meristic and morphometric characters, including number of premaxillary and dentary teeth, number of longitudinal series of scales above lateral line, number of pectoral-fin and anal-fin rays, eye diameter, postorbital distance, and snout length. Finally, notes on reproduction and parasitism for E. correntes are presented.

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A detailed osteological study of the poorly known and critical endangered ghost knifefish, Tembeassu marauna, from the rio Paraná, Brazil, was conducted using X-ray microcomputed tomography (μCT scan). A redescription of the external anatomy was performed, including the unusual presence of a rostral patch of extra teeth on the region of the upper lip anterior to the premaxilla and the prominent anterior fleshy expansions in both upper and lower lips. The newly surveyed characters were included and analyzed in light of a recent morphological data matrix for Gymnotiformes. In spite of some uncertainties that remains as to phylogenetic allocation of the genus, the most probable hypothesis is that Tembeassu is the sister group of a clade that includes Megadontognathus and Apteronotus sensu stricto. The phylogenetic analysis also supports that Tembeassu is considered a valid genus of Apteronotidae. An amended diagnosis for the genus is also provided.