Abstract
During a routine biomonitoring project of the water quality in rivers and brooks in Flanders, Belgium, an unusual Ulnaria taxon was found that could not be identified using the currently available literature. Based on detailed light and scanning electron microscopy observations, the unknown taxon is described as new to science: Ulnaria verhaegeniana sp. nov. The new taxon can be distinguished by its parallel margins, abruptly tapering to the valve apices, its relatively modest valve dimensions with a valve width < 6 µm and the very large, apically elongated, rectangular central area. The new taxon is compared to all Ulnaria taxa presently known. Notes on its ecology are added.