52 Proceedings of the Royal Physical Society. 



of Greenland is similar to that of North Africa." Some 

 of the smaller plankton organisms have probably a world- 

 wide range. In such a limited area as Scotland, it would 

 not, then, be surprising if there were no important variation 

 of the plankton over the whole country. This is far from 

 being the case. The plankton of the extreme north differs 

 greatly from that of the south ; that of the west from that 

 of the east. Scotland appears to be favourably situated 

 for the study of fresh -water plankton, as from its geo- 

 graphical position it forms a meeting place for the northern 

 and southern zooplankton, the eastern and western phyto- 

 plankton. All classes of organisms in the plankton do 

 not exhibit equally this difference of character between 

 the north and the south, the east and the west; it is 

 most marked among the Crustacea, especially of the genus 

 Diaptomtis, and in the Desmids. 



EOTIFERA. 



The pelagic Eotifera of the Scottish lakes form a very 

 small society. There are only about a dozen common 

 species, which are distributed over the whole of the main- 

 land and islands, some of them being the commonest of 

 all plankton organisms; a few rare species are found in 

 one or two lochs. There is no indication of the restriction 

 of any species of Eotifera to definite regions of the country, 

 except the occurrence in many lochs of North Uist and 

 Shetland of Plmsoma hudsoni, a much larger species than 

 the common P. truncatum of the mainland. This apparent 

 limitation, however, may arise merely from insufficient 

 observations of fresh material. 



Crustacea. 



Among the Cladocera, the genus Bosmina includes two 

 species which are dominant in the east and west re- 

 spectively. 



Bosmina oUusirostris, Sars, prevails over the whole of 

 central, west, and north Scotland, and the islands, being in 

 extensive districts the only species ; it is not wholly absent 

 in the east, but is rarer there. 



Bosmina longirostris (0. F. Muller) is the common species 



