1904-5.] The Plankton of Thingvallavatn and Myvatn. 1153 
occurrence of different Desmids in the limnetic region of Thing- 
vallavatn is quite in accordance with the explorations of the lakes 
of the Faeroe islands, hut particularly with those of Scotland. In 
the lakes of the most western part of the European Highlands, 
Scotland, Faeroes, Iceland (with regard to Norway nothing at all 
is known) there seems to exist a peculiar semi-limnetic flora of 
Desmids, which in the other parts of Europe is only represented 
by one or two species ( Staurastrum gracile , S. paradoxum). 
With regard to their occurrence in Scotland, Wesenberg-Lund 
(1905) has supposed that their very peculiar occurrence may he 
accounted for by the fact that these Desmids originally are 
derived .from, and also nowadays are recruited from, the rich 
Desmid flora living in the almost always mist-wrapped moist 
moss carpets which cover the precipitous sides of the hills, where 
streamlets feed the rivers and from whence the Desmids, especially 
in spring, are carried out with the rapidly flowing waters into the 
limnetic region of the lakes ( cf . p. 1158). 
Probably this supposition will also hold good as to the occurrence 
of Desmids in the limnetic region of the lakes in the above- 
mentioned localities, although the hills around Thingvallavatn are 
not precipitous. 
4. With regard to the Peridinians we particularly note the 
total absence of Ceratium hirundinella. In accordance with 
explorations in more southern countries, this was only what might 
he expected, the max. of C. hirundinella lying at a tp. which 
the Icelandic lakes rarely attain. Besides, we refer to the remarks 
about the Euflagellata. 
5. The absence of Dinobryon is more striking. We are in- 
clined to suppose the absence of this genus in the Thingvallavatn 
to be only accidental ; probably Dinobryon is one of the main 
forms in arctic and northern lakes. 
6. The total absence of Tintinnidium and Codonella, whose 
max. occurs at a relatively low tp., is rather remarkable. 
7. The plankton Rotifers belong chiefly to the cosmopolitan 
perennial stock, whose members in more southern countries are 
poly- or di-cyclical, with max. at c. 12° C. in spring and in autumn. 
According to observations from Myvatn, it seems as if the group 
in the more northern lakes is monocyclical, with the max. 
PROC. ROY. SOC. EDIN. — YOL. XXV. 73 
