88 



in the December plankton of 1894, but was exterminated from the 

 channel plankton taken in the following February by the stagna- 

 tion attending the long-continued ice blockade. It reappears in 

 April, and again disappears promptly, but does not return until 

 September 12, and not in numbers until October. There are pulses 

 November 20 (506,800) at 42.8, and December 30 (362,520) at 36.5. 

 The December pulse is followed by a decline, with a rise during 

 February to a well-sustained maximum during March, approaching 

 400,000, and at from 35 to 48. The decline follows in April, and 

 there are only isolated occurrences in small numbers at irregular 

 intervals during the summer. Continuous occurrence begins again 

 in September, and numbers rise rapidly in October. There is a pulse 

 of 542,699 on December 3 at 32.2, and another on March 22, 1897, 

 of 159,500 at 43.8. Synura is very rare indeed in the summer of 

 1897, and in the prolonged low water, sewage contamination, and 

 higher temperatures of the unusual autumn of that year it does 

 not reappear continuously until October 26, at 59, and does not 

 exceed 1,000 until December 7, at 32. There is a low maximum 

 of 98,700 on December 14 at 36, followed by a decline during the 

 rising flood of January-March, 1898. The slight cessations in the 

 flood invasion (Pt. I., PL XII.) in January and in the second weeks 

 of February and March produce prompt responses in immediate 

 rise in numbers in Synura. Finally, a low maximum of 320,600 is 

 attained upon the crest of the March flood, on the 29th, at 49. 

 This is followed by a decline during April and a few scattered 

 appearances during the summer. Synura returns at the end of 

 October and rapidly mounts to a pulse of 1,999,500 on November 

 29 at 35 with the first decline of the November overflow (Pt. I., 

 PI. XII.). A second pulse of 2,764,800 on December 20 at 33, 

 under the ice, gives way to a decline to 51,600 towards the end of 

 January, 1899, during rising water. On February 14 another pulse 

 (348,800) appears at 32.5, under heavy ice, and declines again in 

 the sudden flood of the last days of February, but recovers quickly 

 with a maximum pulse of 898,800 on March 7 at 32.8. Within a 

 fortnight this falls to the low level of 9,800, but its further history 

 was not followed. 



From these data it is evident that in our waters at least Synura 

 is limited to the months from October to April, except isolated and 

 irregular occurrences of small numbers during the summer. Its 



