1904 - 5 .] The Plankton of Thingvallavatn and Myvatn. 1107 
With regard to the physical conditions of the Thingvallavatn 
and the surrounding country, I may refer to the remarks above 
written by Dr Wesenberg-Lund ; hut I must he allowed to 
quote some few words from my little note concerning Icelandic 
fresh- water plankton (1904, p. 235) : “ I think we may say that the 
plankton is like that of the lowland lakes of northern Central 
Europe and Southern Scandinavia, but much poorer, especially 
by the want of the summer forms ; this also is the case with the 
climate ; it is the climate of northern Central Europe and 
Southern Scandinavia with regard to the autumn, winter, and 
spring, but the summer is skipped over.” The insular climate 
causes the lakes, at least the lowland lakes in South Iceland, to be 
ice-covered during only a short period in winter or not at all, 
while the cold summer, with its abundance of cloudy and rainy 
days, is not warm enough to give the lake a high summer 
tp. These conditions will cause a plankton, in which the 
Diatoms predominate all the year round, and the Myxophycese 
are wanting or nearly so, as they reach their max. at higher tps. 
Such is the result of the examinations of the samples from 
Thingvallavatn. In the accompanying table (pp 1154, 1155) 
I have arranged the phytoplankton according to the dates of 
collection, and furnished them with the ordinary signs denoting the 
quantity. We learn from this that the number of species is but 
small, and when restricted to the true limnetic species only, this is 
even diminished by half its number. The species belong to the 
Chlorophycese and Ilacillaricere, to which two Flagellates (Mallo- 
monas and Peridinium ) are to be added. The Myxophycece are 
completely wanting ; but it must be noted that some of the samples 
contain specimens of bottom Myxophycese, viz., Lyngbya sp., 
Anabcena variabilis , Kiitz., but only very sparsely, the few 
specimens occurring being mixed up in the plankton by wind 
and waves. Such accidentally occurring bottom forms are rather 
common in most of the samples ; the climate is very windy, so it 
is but natural that the chance to meet with bottom forms in the 
plankton is great ; especially are bottom Diatoms met with very 
frequently in the samples, viz. Cymbella cistula, Ceratoneis arcus y 
Epithemice , etc. Such forms which are specified in the table, 
viz., Fragilaria construens, F. capucina , Synedra acus, S. ulna y 
