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Mr. W. West and Dr. G. S. West. [Oct. 30, 



In discussing this phenomenon of the rich Desrnid-flora of the British 

 freshwater plankton, it is necessary, in the first place, to briefly outline 

 the general distribution of the Desmidiacese in the British Islands, quite 

 irrespective of the freshwater plankton. We have studied the distribution of 

 British Desmids in detail during the past 16 years, and the obvious fact, 

 patent to anyone who chooses to collect these plants over extensive areas, 

 is t the much greater richness of the Desmid-flora in the western areas of the 

 country. The eastern districts of England are exceedingly poor, but on 

 passing from the newer Tertiary formations to the Older Palaeozoic and 

 Precambrian formations the Desmid-flora gradually increases in richness, 

 attaining its maximum diversity in certain of the Precambrian areas. 



The richest areas of all are the little boggy pools and smaller lakes of 

 the Lewisian Gneiss of North-west Scotland and the Outer Hebrides, and 

 similar areas on the Precambrian formations of Donegal, Mayo, and Galway. 

 There are also several very rich localities in the English Lake District and 

 North Wales, all on the Silurian and Ordovician with sundry Igneous 

 intrusions. There are, in addition, two rich localities in the south of 

 England, one on the Lower Greensand of Surrey (Thursley Common), and 

 the other on the Middle Eocene of Hampshire (the New Forest). In both 

 these localities there are deep, spongy bogs, with a fairly rich Desmid-flora, 

 but at the same time it is a flora which falls far short of the much richer 

 Desmid-floras of the Precambrian areas. We may add that these are not 

 statements based upon a few casual observations, but upon a detailed 

 examination of many thousands of collections made in all parts of the 

 country, from the Shetland to the Scilly Islands, and from the east of 

 England to the west of Ireland. It is also necessary to give some 

 explanation of what is meant by a " rich " area. We do not apply the term 

 " rich " to a mere abundance of Desmids, or even to the occurrence of a 

 great quantity of 30 or 40 species, but only to those areas in which 150 

 to 200 (or even 300) species can be found in more or less abundance, 

 including many of the rare species with a restricted distribution. 



We have, therefore, as a foundation on which to base this discussion of 



possible to be certain of his identifications, are those of shallow alpine and subalpine lakes, 

 and not in any way comparable to the characteristic plankton species of the western 

 British areas. Neither do we regard his records as constituting a " rich " Desmid-flora. 



Among extra-European lakes, Victoria Nyanza has a conspicuous Desmid-flora in 

 the plankton, and the Yan Yean Reservoir, Victoria, possesses a very rich Desmid- 

 plankton, quite equal to the best of the British lakes, although with an entirely different 

 association of species. In the latter case the drainage water is mostly from Silurian and 

 Granitic outcrops, but it is not yet possible to make a definite statement concerning the 

 drainage into Victoria Nyanza. 



