OBSERVATIONS ON NORTH AMERICAN GOMPHONEIS (BACILLARIOPHYCEAE). I. VALVE ULTRASTRUCTURE OF G. MAMMILLA WITH COMMENT ON THE TAXONOMIC STATUS OF THE GENUS1,2

Recent questions concerning the taxonomic status of the diatom genus Gomphoneis Cleve have prompted critical examination of the valvar morphology of a species originally included in the genus. Light and electron microscopic observations on G. mammilla (Ehr.) Cl. show that the characteristics put forth by Cleve to delineate the genus are present in this taxon. Striae composed of two rows of simple areolae located in depressions on the valve and longitudinal lines formed by a broad internal axial plate were observed in G. mammilla. The presence of two apical spines on the headpole and the structure of a bilobed apical pore field located at the footpole are described, in addition to other valve features. Valve morphology of G. mammilla is compared with that of doubly-punctate Gomphonema species with the result that we recommend the two genera remain separate.