A full description and diagnosis are given of Lawrenciella schaefferi Poplin 1984, from the Pennsylvanian (Late Carboniferous) of Kansas, U.S.A. This taxon is only known by braincases and associated parasphenoids, which are exceptionally well preserved as phosphatic nodules. The external and internal morphology of the endocrania were studied from serial sectionings which allowed detailed three-dimensional reconstructions. The study shows that, besides synapomorphies inherent to basal actinopterans, L. schaefferi displays a set of very peculiar features such as prespiracular fossae, intracranial ossicles, and paired anterior myodomes for nonocular eye muscles. Another conspicuous feature, the notochordal canal ending blindly anteriorly, has been observed until now only in more advanced Triassic actinopterans. The anatomical characters of L. schaefferi indicate, in the light of recent cladistic analyses, that this Carboniferous taxon is a relatively advanced actinopteran compared to basal forms such as Kentuckia, Mimia, or Moythomasia.