222 
mum occurs in November—December. Numbers are at a minimum 
generally less than 5,000 per m.*) in midwinter (January—February) ; 
rise in March to about 25,000 per m.?; and attain the maximum for 
the year in a vernal pulse of 200,000 to 1,500,000 in April-May. 
Following this, there is frequently a second pulse of large proportions 
in June, which in 1898 exceeds (Table I.) that of May. During the 
remainder of the year there is usually a series of recurrent pulses, of 
declining amplitude in 1896 and 1898, but rising to unusual heights 
(618,750 on September 9) in the stable conditions of 1897. In 
the main the pulses of Entomostraca coincide with or approximate 
to the location of those of the other organisms of the plankton, and 
often show correlations in amplitude. 
BRANCHIOPODA. 
Eubranchipus serratus Forbes. Young branchiopod larve 
questionably referred to this species appeared in the plankton 
in January—March, 1899, in small numbers at minimum tempera- 
tures. 
CLADOCERA. 
Average number, 6,068 per m.? In 1897 they were more abun- 
dant, averaging 17,863 per m.* in the more stable conditions of that 
year. In 1896,a year of recurrent floods, numbers fell to 7,719, while 
in 1895, a year of low water in spring, when many of the Cladocera 
attain their maximum, the greatest average, 31,937, was recorded. 
The phenomenal number of 443,716 per m.* appeared on June 19 in 
the stable low water (1.80 ft.) then prevailing. In 1894, another 
year of low levels, the annual average was also large (23,952), though 
probably enhanced by the fact that collections were not made in 
flood waters in this year. 
The Cladocera appear in all but 10 of the 182 collections enu- 
merated, the ten exceptions falling in November (1), January (2), 
February (6), and April (1), and usually in flood waters or, as 
in 1895,in stagnation conditions under the ice. Although the 
Cladocera occur in all months of the year, they nevertheless, as a 
group, exhibit decided temperature adaptations, as appears from 
the fact that all records 1n excess of 4,000 per m.' fall between May 1 
and September 1 with but 6 exceptions,—4 in-the phenomenally 
