Abstract
An attachment mode of diatom valves is reported through SEM observations on frustules derived from the Miocene Nabuto Formation. All the frustules examined, which are of centric type either cylindrical or biconvex and consist of two thick-walled valves, and the attachment is completed by abutting their truncated or flanged edges on each other without intercalations between them.
Various types of frustules t h at share this particular mode of attachment have been observed in eleven new taxa (including two new combinations) described under five new genera: Eustephanias, close to Stephanopyxis Ehr., has three concentric rings of rimo portules combined with or without pericentral locking processes. E. rami genus, E. quasinermus and E. inermis are included; Biturricula, monotypic with B. unca, raised a lofty pro cess at the valvar apex to interlock; Dactylacanthis with three new species (D. rara, D. Proxima and D. invalida) forms chains of frustules by tangling their slender processes. In Stephanonycites, to which Sn. variegatus, Sn. coronus and Sn. tabularis belong, the marginal locking spines are hollow and complex but free from rimoportules, unlike the above; and Stictolecanon has a skeletal frame of valves similar to that of Stictodiscus Grey. but develops a deep valve mantle and central rimoportules in the case of Sl. papillatum and Si. geminum.
All but Stictolecanon are grouped into a new family Eustephaniaceae due to their sharing common features: the unique valve attachment by flanged edges, the thick silicified valves with the loculate areolae, the interlinking system and the concentric allocation of the rimoportules.
