Abstract
Infraspecific variation has been investigated in the marine epipelic diatom Hantzschia virgata using light and electron microscopy. Although the variation pattern in this species is complex and to some extent discontinuous, allowing the recognition on phenetic grounds of four or five discrete infraspecific taxa including vars. virgata (= H. insolita), gracilis, leptocephala (= capitellata) and wittii (= intermedia), it is concluded that to split the species would be imprudent. Variation has been found not only in characters such as valve outline and size, or stria density, but also in ‘qualitative’ characters such as raphe, fibula and girdle structure, characters which have often been assumed to be constant within species, genera or taxa of higher rank: previous generalizations about the usefulness of certain characters in distinguishing taxa of a given rank are seen to have been incautious.

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