Abstract In the historical Grunow collection in Vienna, a piece of a broken slide covered with mica contained diatom material from Porto Subzanski, a coastal environment in Croatia. This material was examined in scanning electron microscopy. Several poorly documented or improperly assigned taxa within the Achnanthales, namely in Amphicocconeis and Cocconeis, were re-examined. A new combination of Amphicocconeis and a new species of Cocconeis are proposed. The new species, Cocconeis subzanskiensis, is closely related to Cocconeis placentula, except for the lack of a characteristic hyaline sub-marginal rim on the raphe valve, and is compared with other marine taxa pertaining to the same group. Recently described in tropical environments, Cocconeis coronatoides has been found in the Grunow material.