A hexapodous trackway here described from the Shenango Sandstone equivalent of Elk County, Pennsylvania, is 79 cm wide, by far the largest arthropod trail known. Individual tracks range from subcircular to concave impressions and the opposing members of each pair are in phase. The trackway could have been made by a stylonuroid, drepanopteroid, mycteropoid or hibbertopteroid eurypterid, or by a cyrtoctenid. Information is lacking on the limbs of most of these chelicerates, so no detailed gait analysis is possible. The fragmentary nature of specimens of these morphologically unusual chelicerates, their occurrence in fresh-water to possibly terrestrial habitats, the nature of the eurypterid gill-tract and the present trackway suggest that they were amphibious. The concept of Palmichnium is expanded to include this trackway, and that described by Hanken and Stoermer (1975) from the Upper Silurian of Ringerike, Norway, and both are assigned to new species, P. stoermeri and P. kosinskiorum, respectively.--Modified journal abstract.