Abstract
Frustule structure and symmetry, protoplast morphology, and auxosporulation are described for the raphid diatom genus Petroneis, separated from Navicula in 1990. Several features, such as the constant cis symmetry of frustules, flap-like occlusions of the areolae, the opening of the central external raphe endings into a ‘tear-drop’–like groove, valve-appressed plastids, secretion of mucilage capsules during vegetative cell division, and aspects of sexual reproduction, suggest a close relationship between Petroneis and Lyrella; this is supported by a parsimony analysis. Molecular phylogenetic analysis of selected raphid diatoms, based on rbcL sequences, also indicates that Petroneis and Lyrella are sister genera and supports their separation into the family Lyrellaceae, order Lyrellales. This group is ancient, since Lyrella is known from the Palaeocene.
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