1904 - 5 .] The Plankto7i of Thingvalla.vatn and Myvatn. 1135 
from 23rd January till 31st May, when the first very slender 
young ones may be noticed ; these occur also in the last samples, 
15tli and 30th June. In D . longispina of the Thingvallavatn we 
only find a very slight seasonal variation. The microcephala forms, 
which occur in May, and probably disappear in July-August, are 
very much alike throughout the period, but they differ some- 
what from forms which occur during August- January and which, 
as stated above, may be considered as obtusifrons forms. The 
drawings will show the main differences, especially with regard to 
the shape of the head. I have not been able to point out any 
seasonal variations as to the obtusifrons form. Only in December 
it may be noted that the ephippial females occur in two sizes, the 
one is c. 2-3 mm., the other only 1'2 to 1*5 mm., the former has 
a short spine, the latter commonly a much longer one (figs. 4 and 5). 
From the facts stated above, we may gather that the life cycle 
of D. longispina lasts at least from the end of May to February. 
Still, I think it very probable that some animals — the young ones 
of the last autumn’s brood — here as in Danish lakes survive in 
deep water and propagate in spring. How large. a percentage of 
the total number of D. longispina in the lake the stock of these 
females amounts to, and how many of these individuals are derived 
from ephippia, we are unable to decide. 
With regard to the number of generations and broods, I have 
arrived at the following conclusions. 
At the end of July and in August we find males and females 
with ephippia, but after the 15th September no ephippial females, 
but only young animals with 2 to 3 parthenogenetic eggs and a 
few not quite developed males, occur. As we again find on 30th 
September ephippial females and numerous males, I cannot see 
but that D. longispina in the Thingvallavatn may be regarded 
as dicyclical. 
Moreover, when taking into consideration that during the time 
in which the first sexual period appears we first (July) find females 
with 2 to 3 eggs and later on males and females with ephippia, I 
conclude that the spring generation to begin with will propagate 
parthenogenetically, thus producing males and females, but in all 
probability only one or two broods ; afterwards it will produce 
ephippia and then disappear (September). 
