1904 - 5 .] The Plankton of Thingvallavatn and Myvatn. 1131 
Notholca longispina , Kell. 
N. longispina of the Thingvalla lake differs slightly from 
most specimens of southern countries, the posterior spine is not 
straight or only slightly arched, hut shows a very remarkable 
upward curve. 
It is perennial, and seems to carry its eggs all the year round ; 
strongly marked max. and min. have not been pointed out, but it 
seems to me that it has been most common in winter. 
Notholca striata , O.F.M. 
Only a very few specimens of N. striata have been found ; just 
as with us, it appears in the winter months and disappears in 
early spring. 
Gonochilus unicornis , Rousselet. 
The first individuals were found on 30th August, the last on 23rd 
January. I have not b^en able to find a single individual from 
January to August. In the samples the colonies have all been 
broken up and only single individuals were found. 
Except Ploesoma lenticulare , all the Rotifers mentioned above 
belong to the common, widely spread stock of cosmopolitan plankton 
organisms which have been found in every locality where plank- 
ton explorations have been carried on. Their existence on the 
Kolgujev island has also been pointed out by Skorikow (1904, p. 
209), and on the Murman coast by Levander (1901, p. 1.) It 
is strange that only so few of these species have been mentioned 
by Bergendal (1892) from Greenland, which has been better 
explored as regards its Rotifer fauna than any other arctic country. 
According to my opinion, this stock of cosmopolitan plankton 
Rotifers may also be found in Greenland. The reason why Prof. 
Bergendal has only met with such a few species in his explorations 
of the pelagic region of the greater lakes, is most likely because 
he could not make use of a plankton net, the plankton explora- 
tions in fresh water at that time (1890) being anything but 
methodically carried on. 
