222 



mum occurs in November-December. Numbers are at a minimum 

 (generally less than 5, 000 per m. 3 ) in midwinter (January-February) ; 

 rise in March to about 25,000 per m. 3 ; 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 larvae 

 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. 3 In 1897 they were more abun- 

 dant, averaging 17,863 per m. 3 in the more stable conditions of that 

 year. In 1896, a year of recurrent floods, numbers fell to 7,7 19, 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. 3 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 in excess of 4,000 per m. 3 fall between May 1 

 and September 1 with but 6 exceptions, 4 in the phenomenally 



