Remarks: In 1885 Robert Boldt (Boldt 1885) described Staurastrum arnellii from material that was collected along the Yenisei River on one of the botanical expeditions of the famous polar explorer Nordenskjöld. Later authors (West, G.S. West et Carter 1923 and Coesel & Meesters 2007) associated his description with specimens that are very common in Europe. Peter Coesel (Coesel 2007) saw so many similarities with Staurastrum hirsutum Ralfs that he made S. arnellii a variety of this species, but later he came to the conclusion that the species of Boldt was something different than the specimen commonly encountered in European samples. More than a century later Rupert Lenzenweger (Lenzenweger 1997) made the second reliable find of S. arnellii in Austria. That meant that S. arnelli was suddenly less common in Europe and only known from Siberia and Austria. So it was a surprise to find Staurastrum arnelli also in the Netherlands, incidentally in ... a rain puddle in Amsterdam!