1908.] The British Freshwater Phytoplankton 



201 



In reference to the above remarks, James Murray* has stated that 

 " another theory is that the lochs which are richest in Desmids are only 

 found in the older geological formations, but this does not accord with facts, 

 as I find that such lochs occur in all the formations from the Lewisian to the 

 Tertiary, at least ; and it will, I think, be found that some of these lochs lie 

 entirely in glacial deposits." It should be pointed out that if glacial drift is 

 excluded none of the lake-areas are even near the British Tertiary formations. 

 Many of the lakes have doubtless been formed in Tertiary times, and if they 

 are spoken of as " Tertiary lakes " it must be distinctly understood that they 

 are situated in drainage-basins on the old formations. Those lakes which 

 Murray has termed " Tertiary lakes " are certain of the Scottish lochs which 

 are surrounded by and rest upon more or less extensive sheets of glacial 

 drift. It must be distinctly borne in mind that most of this glacial drift 

 has been derived from the old rocks, and what is very much more important, 

 that the drainage into such lakes consists mostly of water which has partly 

 'traversed the exposed outcrops of the old rocks of the surrounding hills and 

 mountains, and partly percolated through them. Therefore, any peculiarities 

 which may be characteristic of the drainage water from the old formations, 

 are also equally characteristic of the water of such lakes. 



The point of primary importance is that the greater part of the drainage 

 water of such a lake has traversed the older rocks, and possesses those 

 peculiarities which so far as we can see account for the Desmid-flora not 

 only of its plankton, but of its littoral region and also of the surrounding 

 bogs. It is immaterial when the lake was formed, or whether its bed be one 

 of glacial drift or of old rocks. 



In the British Islands the really rich Desmid-flokas, containing 

 many of the western british types, are only found in those areas 



WHICH COMBINE THE MOST SUITABLE HABITATS (such as are found Oil boyyy 



hill-sides with an abundant rainfall) avith a drainage-water derived from 



GEOLOGICAL FORMATIONS OLDER THAN THE CARBONIFEROUS. 



We would suggest a very exact chemical investigation of the water of bogs 

 and lakes in different areas as a possible means of throwing further light 

 upon this question. It has been suggested that the absence of lime is the 

 determining factor in the abundance of Desmids, and it may possibly have 

 much to do with the restricted distribution of the western British types. 



There are a large number of western types of British Desmids, of which 

 the most important are included in the following list : — 



Gonatozyyon aculeatum, Hastings ; Spirotamia trabeculata, A. Br. ; Peniiom 



* James Murray, loc. cit., 1905, p. 58. 



