234 



DREISSENSIA POLYMORPHA, PALLAS. 



HANS SCHLESCH, 



Hellerup, Denmark. 



Dreissensia polymorpha is one of the few species of which 

 it is possible to trace the introduction into the parts of Europe 

 in which it occurs. According to Mousson and v. Martens 

 its original home is S. E. Europe, as it is found in small lakes 

 isolated from rivers and canals in Rumelia and Albania, and 

 Andrussov records it from the tertiary formations near Bejuk- 

 Schor and Balachany in Caucasia*. Dreissensia polymorpha 

 was first found by Pallas in 1768 in the lower parts of the 

 Ural River |) and it was found in the Wolga in 1780. It is 

 interesting to note that this species lives as well in salt as 

 in freshwater, and on account of its resemblance to Mytilus 

 Pallas named it M . polymorphus. It inhabits also the Caspian 

 Sea, Lake Aral, etc. From these parts of Europe it has 

 spread over the greater part of Europe, except Scandinavia, 

 Spain, Portugal, Italy and Greece, fastening itself by its 

 byssus to ships, etc., and proceeding up the rivers, has passed 

 from one river valley to another, until it reached the Baltic. 

 On 2nd November, 1824, Sowerby exhibited in the Linnean 

 Society, the first British specimens, found in the London 

 Docks. In 1825 it was recorded from Frische — and Kurische 

 Haff, in 1826 in the Rhine near Leyden, in 1827-29 in Havel 

 near Berlin ; in 1835, in the Eidern in Holstein and 1843, by 

 Morch from Copenhagen. From Holland, it has spread over 

 Belgium and France and in Germany down the Rhine, as far 

 as Basel, and the Main river, and by the Danube — Main Canal 

 into the Danube to Regensburg. 



Hazay records Dreissensia polymorpha from Budapest, in 

 1824, but probably it was carried by ships from the Black Sea. 

 At the present time, it probably inhabits the Danube along its 

 whole length. An example of an isolated locality in which it 

 has suddenly appeared, is Lake Fureso, a few miles north of 

 Copenhagen. Fureso is certainly the best studied locality in 

 Denmark since the days of the celebrated Danish malacologist, 

 Otto Friederich Miiller, who did much collecting there, but it 

 was found there first only a few years ago, and in abundance. 

 In April, 1835, Prof, van Beneden named this species Dreissen- 

 sia, in honour of the apothecary, Dreissens, who obtained the 

 first specimens in the Maastricht Canal ; at the same time Prof. 

 Rossmassler named it Trichogonia, but as van Beneden also 

 described the animal, the name Dreissensia is to be used. 



* A portion with fossil Dreissensia is to be seen in the Hull Museum. 

 t Reise durch versch. Prov. des Russ. Reich's 1771, I., p. 375. 



Naturalist 



