318 Birge — The Crustacea of the Plankton. 



winter of 1896-97 Aphanizomenon and Diatoma were present, 

 together; the latter form being more abundant at the opening 

 of the winter and the former relatively increasing towards 

 spring. Asterionella has been regularly present in all years as 

 a small part of the summer plankton, but never has been predomi- 

 nant except during a short time in the spring of 1897. Ceratium 

 has been a leading alga in the summers of 1895 and 1896, but in. 

 1894 and 1897 there was no Ceratium period. Lyngbya predom- 

 inated in July, 1895, but scattered filaments only were present, 

 during the succeeding two seasons, while in August and Sep- 

 tember, 1897, it was again present in considerable numbers,, 

 though nowhere near as great as in 1894. The summer of 1895. 

 was definitely a diatom season, as was also that of 1897, very 

 few of the Schizophyceae being present; while in 1896 the latter 

 plants predominated, although a considerable number of dia- 

 toms were always present. In the autumn there has always- 

 been a diatom period, but the predominant forms have been 

 Diatoma, Fragillaria, and Melosira in different seasons. The- 

 first alga to develop in the spring is one of those which has 

 predominated during the winter, but the order of succession 

 in the forms which follow is wholly uncertain, as the few illus- 

 trations given above sufficiently indicate. 



LARGEST NUMBER OF CRUSTACEA PER CUBIC METER. 



The following list shows the largest number of Crustacea foundl 

 per cubic meter. It is computed on the assumption that the ani- 

 mals are equally distributed through the three meter space cov- 

 ered by each haul of the net and gives the average per cubic 

 meter for the distance of three meters. In reality the maximum 

 at the stratum of greatest abundance would be greater than the 

 table shows. Probably 600,000 would not be too high as 

 the maximum for the total number in a cubic meter. The 

 numbers are given as thousands per cubic meter. All, except, 

 D. pulicaria are from the upper, or 0-3 meter level. 



