10 DR. С. $. WEST ON THE ALGÆ OF 
abundant and noteworthy forms are discussed in connection with their 
periodicity (consult page 19). 
I ascribe the richness of this Desmid-flora to the nature of the geological 
formations upon which the Yan Yean Reservoir is situated, and from which 
it derives its water-supply. During the past sixteen. years I have made a 
detailed study of the Desmidiace:e, and, so far as Western Europe is concerned, 
the distribution of characteristic types, and also the richness of the Desmid- 
flora, has a close relationship with the geological formations of the drainage 
areas which may be under consideration *. The really rich Desmid-floras 
are only found on the Older Paleeozoic and Precambrian formations T. The 
phytoplankton of lakes in these areas is much richer in the Desmidiaceæ than 
that of lakes situated on newer formations. Even the lakes of a Carboniferous 
area are poor as regards their Desmid-flora, unless part of the drainage is 
derived from outerops of Silurian or of still older rocks. Outerops of Igneous 
material also materially increase the richness of the Desmid-flora of a district. 
It seems probable that one of the primary factors which brings about this 
richness of the Desmid-flora is the hardness and durability of the rocks con- 
stituting the older strata. The resistance of these rocks to subaérial denu- 
dation is due in a large measure to their antiquity ; in other words, to the 
great compression they have been subjected to owing to weight of overlying 
strata, and to those movements in the earth's erust which have produced 
folding and contortion. The direct outcome of this action has been the pro- 
duction of mountainous regions which are suitable for the formation of peat- 
bogs ; and wherever these conditions have been realized, there the Desmids 
flourish very well. They unquestionably thrive in suitable situations where 
the water contains small quantities of humic and other organie acids ; but it 
seems probable that other chemical considerations play no small part in their 
distribution, otherwise it 1s difficult to account for those types which are appa- 
rently limited to the bogs and lakes of areas older than the Carboniferous. 
With the one exception of Melosira granulata, the Diatoms are not very 
conspicuous. The complete absence of Fragilaria and Asterionella is а note- 
worthy feature $. The comparative scarcity of Diatoms and the absence of 
some of the widely distributed plankton-genera is to be attributed to the 
relatively high temperature of the water, which was not below 10° C. at the 
time of collection on any occasion during the thirteen months over which the 
observations were made. Many of the European plankton-diatoms attain 
* This relationship is at present being worked out in detail in the British Islands, where 
we are particularly fortunate in having almost a complete succession of geological strata. 
+ W. West & G. 5. West, “The Alga-flora of Yorkshire," Trans. Yorks. Nat. Union, v. 
(1901) p. 9; G. 5, West, Treatise Brit. Freshw. Algæ (1904), p. 6. 
{ The genera Fragilaria and Asterionella were entirely absent from the plankton of the 
large African lakes in which the mean temperature of the surface-water was above 20° C. 
Cf. G. S. West in Journ. Linn. Soc., Bot. xxxviii. (1907) pp. 84 and 88. 
