Beschreibung:
<sec>
<label>Background and aims –</label>
<p>During a survey of the soil diatom flora of the sub-Antarctic islands in the southern Indian Ocean, an unknown small-celled naviculoid taxon was discovered living in small cracks in cliffs bordering the ocean. The taxon showed a set of morphological features only found in the genus <italic>Microcostatus</italic>. However, the unknown taxon could not be identified using the currently available literature.</p>
</sec>
<sec>
<label>Methods –</label>
<p>Using both light microscopical and scanning electron microscopical techniques, the morphology of the unknown <italic>Microcostatus</italic> taxon was documented. The new taxon is described, illustrated and compared with all other similar <italic>Microcostatus</italic> taxa worldwide.</p>
</sec>
<sec>
<label>Key results –</label>
<p>
<italic>Microcostatus elisabethianus</italic> possesses a unique combination of morphological features that excludes conspecificity with all other members of the genus. It is characterized by a lanceolate valve outline with protracted, elongate apices, a very large, porous conopeum reaching the valve margins and uniseriate striae composed of only one macroareola and mantle areolae restricted to the valve apices. The morphological features of the genus <italic>Microcostatus</italic> are evaluated and discussed. Notes on the occurrence and ecology of the new taxon are added.</p>
</sec>