A historical review of the ostracode Bairdia oklahomaensis, s.l., recalls that the species is especially common among Pennsylvanian faunas of the Illinois basin and the midcontinent but that many concepts under at least 4 names have been applied to this species. Earlier confusion resulting from the traditional typological approach to Bairdia does not permit us to accept safely any concept except the all-inclusive one. The species, therefore, had been assigned a very long range, Morrow into Missouri, and had little or no stratigraphic value. The triangular graph is a useful tool for the interpretation of a quantitative study of 12 populations of Bairdia oklahomaensis, s.l., from throughout the vertical and lateral range of this species in Indiana. Vertical speciation occurred, and so B. oklahomaensis, s. s., and B. dornickhillensis are recognized separately with respective ages of post-Atoka and pre-Des Moines and with further refinements in each of their stratigraphic ranges. B. auricula and B. seminalis (Payne only) should be considered as synonyms of B. oklahomaensis, s. s. Geographic speciation or subspeciation apparently did not occur within the broader concept of the species. The triangular graph shows further that the considerably different mean sizes among the several populations are of no genetic or taxonomic significance. The ontogeny of Bairdia oklahomaensis, with 9 molt stages, does not differ importantly from that of modern species of Bairdia. The more recent name Nesidea, sometimes substituted for Bairdia, should be considered as a synonym.