1092 Proceedings of Royal Society of Edinburgh. [sess. 
A Regular Fortnightly Exploration of the Plankton 
of the two Icelandic Lakes, Thingvallavatn and 
Myvatn. By C. H. Ostenfeld, Inspector of the Botanical 
Museum, Copenhagen, and Dr C. Wesenberg-Lund. Com- 
municated by Sir John Murray, K.C.B., F.R.S. (With 
Three Plates.) 
(Read July 17, 1905.) 
I. INTRODUCTION. 
During the last five years the greater part of my time has been 
taken up by the study of the plankton in the fresh-water lakes of 
Denmark, and in the spring of 1904 the first part of an extensive 
paper upon this subject was published by me. In that paper I 
have compared the results arrived at by me with the results of 
plankton explorations in other countries, and have tried to collect 
together everything known at the present moment regarding the 
periodicity and geographical distribution of the European fresh- 
water plankton. 
At the time I wrote my paper, extensive and regular plankton 
explorations were carried on no further north than in Denmark. 
On the other hand, plankton explorations had been commenced in 
Holstein and North Germany at an earlier period than mine. As 
the distance between these places, geographically speaking, is 
inconsiderable, it has been impossible for me to ascertain whether 
the differences and similarities between my observations and those 
arrived at further south were due only to the northerly situation 
or to other facts. This is especially the case with regard to the 
plankton of some of the northern lakes in Jutland. 
Very little is known about the plankton in lakes in the 
northern parts of the temperate zone, as well as in arctic regions. 
Regular fortnightly explorations in lakes further north than the 
Danish ones are non-existent. The knowledge obtainable with 
regard to the fresh-water plankton of higher latitudes was restricted 
to an examination of some few samples of phyto- or zoo-plankton 
