226 



Alona quadrangularis O. F. Miill. Average number, 5. A few 

 scattered occurrences in March-May. 



Alona spp. It is probable that some of the foregoing species of 

 Alona are here included. There are 16 occurrences, scattered 

 through all months but January, April, and November, with no 

 large numbers and no marked seasonal distribution. 



Bosmina longirostris O. F. Mull. Average number, 2,441, of 

 which 1,527 are adult females without large embryos, 390 with 

 them, and 524 immature. 



I include in this species B. cornuta Jurine, for I am unable to find 

 any constant line of demarcation between these forms. The 

 longirostris form is the dominant one in the channel plankton, the 

 cornuta form being relatively rare. 



Bosmina is a perennial planktont in our channel plankton, but 

 occurs in small numbers only in October-May, no record in this 

 period with the exception of that of October 5, 1897 (20,400), at 

 71, exceeding 5,000 per m. 3 , and most of them falling below r 2,000. 

 The records in November-March, with the exception of November- 

 December, 1897, all fall below 1,000 per m. 3 In like manner the 

 percentage of collections containing Bosmina in December-April is 

 lower than that in the summer, the percentages being 64, 16, 26, 47,. 

 and 55 per cent, respectively for these colder months, and averaging 

 82 per cent, for the rest of the year. The percentage of occurrences 

 in October-November remains high (82 and 81 per cent.), though 

 the numbers per m. 3 fall off greatly. 



The usual seasonal distribution is as follows : In January-March 

 the occurrences are scattered and irregular and the numbers very 

 small less than 500 per m. 3 Toward the close of April the vernal 

 increase makes its appearance, continues slowly through May, rarely 

 attaining more than 5,000 per m. 3 , and at the end of this month or 

 early in June reaches the maximum development of the year in a 

 vernal pulse of 40,320 (1896) or 62,800 (1898) per m. 3 From this 

 summit there is an abrupt descent in a period of exhaustion to a 

 level of less than 2,000 per m. 3 in the last fortnight of June. During 

 the remainder of the year there appears a series of recurrent pulses 

 of less magnitude, exceeding 10,000 per m. 3 in but three instances. 

 These follow at intervals of four to six weeks. In July-September 

 the amplitude of these pulses exceeds in all cases 5,000 per m. 3 In 

 October (with the exception of 1897, when temperatures were un- 



