236 



SEASONAL DISTRIBUTION OF CHYDORUS. AVERAGE NUMBER PER M. 3 continued. 



occurring in average temperatures, for these months, of 60.5 and 

 68.3, while the minor autumnal development appears in September- 

 October at 74.2 and 57.6 respectively, and the December pulse, if 

 indeed it be a separate and independent pulse, is at the low tempera- 

 ture of 35.2. The December movement may be simply the result 

 of the more stable conditions which attend the appearance of the 

 ice-sheet on the approach of winter. 



An analysis of the course of the seasonal distribution of Chydorus 

 in channel waters, as given in Table I. and in statistics of other 

 years, indicates the following seasonal regimen. In January-Feb- 

 ruary, at minimum temperatures, the occurrences are irregular 

 (75 and 40 per cent.) and the numbers small (average, 167 and 53 

 per m. 3 ), while in March, with rising temperatures, occurrences are 

 more numerous (100 per cent.) and numbers rise to 668 per m. 3 In 

 April-May a high percentage of occurrences (82 and 90 per cent.) 

 continues, and they mount rapidly to the maximum record of the 

 year, which in our statistics varies from 4,088 in 1895 to 32,800 in 

 1897. This vernal pulse reaches its maximum in our records on 

 April 29 in 1895, at 64, and in 1896 on the same day, at 70; on 



