1904 - 5 .] The Plankton of Thingvallavatn and Myvatn. 1133 
Cladocera. 
Daphnia longispina, O.F.M. 
With regard to the longispine group of the genus Daphnia , I 
have in my plankton work (1904) followed Lilljeborg (1900), who 
refers all the longispine Daphnids to the two species D. hyalina 
(Leyd.) and D. longispina (O.F.M. ). Still, I supposed these two 
species to be only one, of which the hyaline group, at any rate in 
our country, mainly consists of plankton organisms in the greater 
lakes, the longispine group belonging chiefly to the central parts of 
ponds and smaller lakes ; for want of time I was prevented from 
studying the smaller lakes more closely, and did not venture at 
that stage of my explorations to refer all longispine Daphnids to 
one species only. Later on Sars (1903, p. 8) has united the two 
species, and Ekman (1904, p. 17) likewise. The form which 
occurs in the Tliingvallavatn proves to belong to the microcephala 
galeata group (Ekman, p. 123). 
In early spring we only find forms which undoubtedly are identic 
with the microcephala forms ; Ekman also classifies these as spring 
forms. In Thingvallavatn the species never develop into the 
galeata forms, which often occur in Sweden as well as in our 
country. In the summer and fall the individuals may most 
probably be referred to the form obtusifrons , yet differing from 
this by a more acute rostrum and a greater bend in the ventral 
edge of the head. 
On 14th July we only find young parthenogenetic mothers, some 
of them with 2 to 3 eggs (fig. 1) ; from 31st July males and females 
with ephippia (fig. 2) occur in all the samples till 15th September ; 
the joint number of individuals is very small. On 15th 
September D. longispina is common; most of the animals are 
parthenogenetic females with few (2-3) eggs ; ephippial females 
do not occur, but some males with faintly developed first pair 
of antennae. On 30th September D. longispina is very common, 
and many of the females have ephippia again ; besides, we 
find many quite young females without eggs of any kind (fig. 
3) ; the males are now common. On 15th October D. longispina 
is remarkably rare ; young females without ephippia and young 
males occur. From 31st October to 14th December D . longispina 
