Muscular articulations on the brachials and pinnules of Paleozoic crinoids have traditionally been inferred by the presence of a transverse ridge. We here provide evidence that the microstructure of the stereom, fine meshed where muscles once inserted and coarse meshed where ligaments were emplaced, provides a reliable microstructural criterion for muscular articulation on well-preserved arm ossicles. Partially pyritized ossicles of Aesiocrinus magnificus Miller & Gurley from the Lane Shale of Pennsylvanian age at Kansas City, Missouri exhibit well-preserved stereom in three dimensions under the scanning electron microscope. These brachial and pinnular plates show clearly the distinctiveness of stereom at sites of muscule and ligament attachment. This criterion should be useful in tracing the evolutionary development of flexible arm articulations in several lineages of Paleozoic crinoids.