The adder’s-tongue ferns, Ophioglossum (Ophioglossaceae) are noted for taxonomic problems. Of the seven species known in southeastern United States, Ophioglossum nudicaule L. fils is the most variable and taxonomically confused. C.M. Allen’s recent discovery in Louisiana of plants matching the tropical American O. ellipticum Hook. & Grev. led to a study of this taxon and its relationships with O. nudicaule. Taxon ellipticum differs from the traditional concept of O. nudicaule in its larger size, complex venation pattern with small secondary areoles included within the large primary areoles, and a pale central band on either side of the midrib on the abaxial surface of the blade. Comparison of forms of O. nudicaule demonstrates that there is a continuous morphological series running from very small plants with relatively simple veins and no included areoles to large plants with complex veins and numerous included areoles. The epithets that have been applied to the United States members of this series are enumerated here, and all of them, including ellipticum, are synonymized under O. nudicaule. According to this new interpretation, Ophioglossum dendroneuron E.P. St. John is an intermediate form that ties together the two ends of the spectrum of variation. A key to all of the species now recognized in the southeastern United States is presented, together with suggestions for making useful collections of these plants. It is hoped that the Allen locality will be preserved for future studies of what is the only presently known adder’s-tongue in the United States that has the characters of typical O. ellipticum.