282 THE FRESH-WATEK LOCHS OF SCOTLAND 
Staurastrum jaculiferum^ S. lunatum^ S. paradoxum^ S. gracile^ 
Hyalotlieca mucosa ; Tabellaria fenestrata^ T. fiocculosa^ Asterionella 
formosa^ A. gracillima^ Surirella rohusta. 
Of the Peridiniaceae only Ceratium Mrimdinella can be definitely 
set down as " general,"" though a species of Peridinium {P. tahidatum ?) 
is very frequent. 
The 2 species of Asterionella are indicated as general, but this 
is rather misleading, as neither species is distributed all over the 
country. Messrs West are inclined to regard both forms as extremes 
of the same species, and it is intended to indicate that Asterionella^ 
in one form or other, is in almost every loch. 
Dinobryon is found in most lochs, but no one form is " general." 
Mallomo)ias is probably " general,'"' but if so, has been frequently 
overlooked, as might from its small size be expected. 
There are thus altogether only some 29 organisms, or equal 
numbers of animals and plants, which can be regarded as generally 
distributed in the Scottish plankton. These figures leave out of 
account certain groups of phytoplankton, at present under study. 
Few of these species ever become dominant in the plankton, to 
the extent of colouring the water or rendering it turbid. Several of 
the Crustacea {D. gracilis^ C. strenuus^ Holopediwn, Daplinia^ Bosmina) 
may do so. 
Of the Rotifers, Aspkmchna, from its large size, most readily 
dominates the plankton, but has only rarely been observed to do so. 
Notholca longispina has on occasion rendered the collections brick- 
red, and somewhat affected the transparency of the water. Anurasa 
coclilearis and Polyarthra platyptera have, in small lochs, contributed 
considerably to the turbidity. 
Ceratium hirundinella is frequently the dominant organism for 
a season, and occasionally renders the water turbid. No one species 
of Desmid is ever conspicuously dominant, but often an aggregation 
of many species, all abundant, gives the character to the plankton. 
The organisms most commonly greatly predominating in the 
water, so as to produce the phenomenon of " flowering," are not 
found among the most generally distributed species. Those are 
among the casual species, chiefly Myxophyceae {Anahosna^ Clathrocystis, 
etc.), or less commonly Chlorophyceas {Volvooc^ etc.). 
Seasonal Vajiation. — The seasonal variation in the composition 
of the plankton is, on the whole, but slight ; although, as it is chiefly 
exhibited among the largest Crustacea, it produces conspicuous 
differences. The four commonest plankton Crustacea, Diapiomus 
gracilis, Cyclops strenuus^ Bosmina obtusirostris^ and Daphnia liyalina^ 
remain all the year round in Loch Ness, and have been frequently 
taken in winter in smaller lochs, though in some of these they may 
