Pelagic Organisms in Scottish Lakes. 55 



Both D. laticeps and D. ivierzejsJdi are undoubtedly much 

 commoner in the districts in which they occur than the 

 figures in the accompanying Table would indicate ; but in 

 a great many lochs only immature animals were obtained, 

 and in that case it is impossible to tell to which species 

 they belong, though it is known that it must be one of 

 those two. This is also true of B. laciniatns, though that 

 is recognisable at an earlier stage than the others. 



Phytoplankton. 



Messrs W. and G. S. West have studied the algse of the 

 plankton in a large number of the Scottish lochs. Much 

 of their work is not yet published, and the published papers 

 do not deal with a sufficiently wide area to enable us to 

 take a comprehensive view of the distribution of the phyto- 

 plankton in general; only in the case of the more con- 

 spicuous of the Desmids of the plankton, where their work 

 has been supplemented by the observations of the Lake 

 Survey, are we able to review the distribution over the 

 whole country. Messrs West have pointed out the extreme 

 richness of the Desmid-flora of the Scottish plankton, and 

 have shown that the most conspicuous species in it are 

 of a distinctly western type, being found in Europe only 

 in the western portions of the British Isles, Scandinavia, 

 and in some cases Lapland; while in N"ortli America they 

 are frequent in the eastern parts of the United States and 

 in Nova Scotia. In the appended Table I have only traced 

 the distribution of a few of the Desmids of this western 

 type, as they alone present features of interest. 



Micrasterias wallachii, Grun., though not cited by Messrs 

 West as belonging to the western type, is included, as it 

 is a rare plant, and its distribution is therefore of interest. 

 It has been found in seven lochs, — Lochs Burialand and 

 Littlester, in Shetland; Lochs nan Cuinne, Leum a 

 Chlamhain, and Euar, in Sutherland; Lochs Bhaic and 

 Nan Eun, in Perthshire. 



In preparing the accompanying Table to illustrate the 

 distribution of some Crustacea and Desmids, I have tried 



