226 
Alona quadrangularis O. F. Mull—aAverage number, 5. A few 
scattered occurrences in March—May. 
Alona spp.—lIt 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.*, and most of them falling below 2,000. 
The records in November—March, with the exception of November— 
December, 1897, all fall below 1,000 per m.* In lke 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.° 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.* Toward the close of April the vernal 
increase makes its appearance, continues slowly through May, rarely 
attaining more than 5,000 per m.’, 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.’ From this 
summit there is an abrupt descent in a period of exhaustion to a 
level of less than 2,000 per m.* 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.* 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.? In 
October (with the exception of 1897, when temperatures were un- 


