1904 - 5 .] The Plankton of Thingvallavatn and Myvatn. 1103 
the first mentioned being a coast station, the second a much 
colder inland station. With regard to Myvatn, there was a similar 
coast station at Akureyri, and a very cold inland station at 
Modrudalur. The mean tp. of the months in which the plankton 
samples have been taken has, according to my request, been 
used for the calculation. I take the liberty to offer Mr Willaume 
Jantzen and the Institute my best thanks for their kind services 
in this respect 
As it was very difficult for me to find the widely spread 
literature treating on Myvatn, I begged of the well-known Ice- 
landic geologist, Prof. Thoroddsen, to give me his aid. He kindly 
provided me with a list of literature appertaining to this lake. 
The sketch of the physical and natural conditions of the lake is 
based upon this list, more especially on Prof. Thoroddsen’s 
own valuable papers on this subject. 
I beg to forward to Prof. Thoroddsen my very best thanks for 
the valuable assistance he has given me. 
For the beautiful photo of Thingvalla lake, my best thanks 
are due to Mr C. Y. Prytz, professor at the Royal Veterinary and 
Agricultural School in Copenhagen. 
C. Wesenberg-Lund. 
II. THING V ALL AY ATN. 
(a) General Remarks. 
With regard to the Thingvallavatn, Mr B. Soemundsson, in the 
journal edited by the Royal Danish Geographical Society 1904, has 
published an account of a thorough bathymetrical exploration of 
the lake, accompanied by a map, and some remarks pertaining to 
the environments, tp., vegetation, and geology. From this paper 
I have obtained the following information. 
The Thingvallavatn is situated in the south-western part of 
Iceland, at c. 64° north latitude. It is a combination of a lava 
and a glacial lake. The length of the lake is about 16 kiloms., the 
greatest breadth is 8 kiloms. The water covers an area of about 
115 square kiloms., the greatest depth is about 110 m., the surface 
of the lake is 106. m. above sea-level, the mean depth is about 
