1138 Proceedings of Royal Society of Edinburgh. [sess. 
thorough exploration, has given a rather similar sketch of the life 
cycle of D. minutus in Green lake and in Lake Winebago. 
According to Marsh, D. minutus occurs there from July to 
December; the great max. occurs about the 1st of August, but it 
also has another but smaller max. in October ; the species is rare 
in winter and spring. 
Cyclops strenuus, Fischer. 
The Cyclops species of the pelagic region of the Thingvallavatn 
is C. strenuus (Fischer), and not C. scut if er (G. 0. Sars), to which 
result Richard (1892, p. 310) already has arrived. The species 
appears in the forma vernalis , Lilljeb. (1901, p 47), which, accord- 
ing to Ekman (1904, p. 30), also is characteristic for the North 
Swedish alpine region. As far as my experience goes, it has no 
conspicuous seasonal variations in the Thingvallavatn. 
On 14th July the species is rare, and only a few young unripe 
animals are found ; on 31st July the number of animals is enormous, 
but all are quite young. In all the following samples till 30th 
September, its occurrence is rare, after that the species again becomes 
common ; ripe males and a few females with eggs appear ; the 
number increases steadily until 23rd January, when the species 
attains the main form and occurs in enormous numbers. All the 
females carry eggs, and males may be noticed in all the samples, 
but become rare in January. Then C. strenuus disappears entirely, 
and is not seen again until 16 th April. In all the following 
samples till 30th June we find the number of individuals in- 
creasing. In May we only observe nauplii or very young animals ; 
they become very numerous in June. 
From the above statements it is impossible to arrive at any con- 
clusion as to the propagation of C. strenuus in the Thingvallavatn. 
The very small number of individuals at 14th July, the enormous 
quantities at 31st July, and the almost total disappearance from 
31st July to 30th September, clearly show that the samples by no 
means verify the existing facts. We are only able to note that 
from 14th July to 30tli September 1902, as well as from 16th 
April to 30th June 1903, no ripe males or females with eggs 
appear. During this period we only find nauplii or half-grown 
