Specimens of a small diatom from Grand Traverse Bay, Lake Michigan, were studied with light and scanning electron microscopy. They were assigned to the genus Amphora Ehr. ex Kutz., although previous investigators had most often treated the diatom as Cymbella parvula Krasske or Cymbella thumensis (Mayer) Hust. Characteristics influencing the genus assignment include differential mantle development, multiple intercalary bands, and raphe structure. Pertinent literature is reviewed. Comparisons are made to Amphora coffeiformis (Ag.) Kutz. isotypes, and arguments for use of the name Amphora thumensis (Mayer) A.Cl. are given.