Pelagic Organisms in Scottish Lakes. 57 



The most important fact which emerges from a study 

 of the distribution of the various species included in the 

 Table, is that many of them occupy areas which coincide 

 approximately. Diaptomus laticeps, D. laciniatus, and the 

 Desmids of the western type, alike extend over the whole 

 of Scotland north of the Caledonian Canal, and into the 

 Outer Hebrides ; south of the Great Glen they are confined 

 to the west coast and some of the central counties, being 

 entirely absent, so far as we know, from all the eastern 

 counties south of the Moray Firth. All have their eastern 

 limit in some small lochs about the extreme western corner 

 of Aberdeenshire. There are some minor differences in the 

 details of distribution — D. laciniatus is absent from the 

 Orkneys, Shetland, North Uist, and Barra; it extends a 

 little farther south than D. laticeps ; the western Desmids 

 extend still a little farther south, to the shores of the 

 Solway Firth. 



Having regard to the distribution of plankton, therefore, 

 Scotland may be divided into two regions — (1) northern 

 and western, characterised by several species of Diaptom%is 

 and many species of Desmids ; (2) eastern, characterised by 

 the absence of those species. 



The eastern region, it may be noticed, is principally low- 

 land, the western mainly highland ; but the chief distinction 

 between them is not merely that of alpine and lowland, as 

 large districts of the north, where the northern Crustacea 

 and the western Desmids attain their maximum develop- 

 ment, are lowland in character. 



Although the peculiar Desmids and Crustacea are associ- 

 ated in the same area in Scotland, the Desmids are at the 

 eastern limit of their range, the Diaptomidse at the 

 western — the species of Diaptomus of the two sides of 

 the Atlantic being entirely distinct. D. laciniatus extends 

 into Scandinavia and the extreme north of Eussia, and is 

 also found in the alpine lakes of Switzerland. D. wierzejskii 

 has been found in many places sufficiently far apart — 

 Madrid, Saxony, the Kola Peninsula. D. laticc2os is found 

 in Scandinavia. 



The peculiar association of species belonging to so many 

 VOL. XVI. G 



