Obeliscus agassizi Pilsbry, 1906 was described from specimens collected in Brazil, by J. G. Anthony, during the Agassiz expedition, in 1865. The type locality of the species was given simply as Brazil, without further information. More than a century after its description, this species was rediscovered during an expedition to the Biological Reserve of Pedra Talhada, Alagoas/Pernambuco states, Brazil. In the present study we provide detailed description of the anatomy of the soft parts of O. agassizi specimens, collected during this expedition, besides more precise data concerning the species occurrence. This work presents the first anatomical description for the genus. An annotated checklist of Obeliscus Beck, 1837 species is also provided. The species of Obeliscus have been traditionally distinguished by shell characteristics, mainly the general shell shape, number of whorls proportionally to the shell size, shell ornamentation, the shape of the protoconch and aperture size and shape. The distinction between species is often subtle. Considering that shell traits may be not sufficient as single taxonomic characters, other evidence for species boundaries, especially anatomical characters, is needed. Presently, it is not possible to discuss the taxonomic meaning of the set of anatomical characters described for O. agassizi, at the intra-genus level, as there is no information on the anatomy of the soft parts of other species of Obeliscus. The comparative analysis of the genital system of O. agassizi, Neobeliscus calcareus (Born, 1780), Stenogyra terebraster (Lamarck, 1822) and Rectobelus birabeni (Hylton-Scott, 1946) pointed out anatomical characters with potential diagnostic significance for the genus Obeliscus. However, the establishment of a definitive differential diagnosis for this genus, based in both conchological and anatomical traits, depends on the analysis of a greater number of species of Obeliscus, as well as species of other genera of Obeliscinae Thiele, 1931.