237 



May 25 in 1897, at 66. 3; and on May 24, in 1895, at 73. From this 

 maximum the pulse declines abruptly in a fortnight to a midsummer 

 minimum during maximum temperatures, which continues until 

 September. During this period the numbers are small, rarely rising 

 above 400 per m. 3 (average, 388, 103, and 49), and the occurrences 

 are also less numerous (72, 61, and 33 per cent.). With the decline 

 of temperatures which begins in September the percentage of occur- 

 rences mounts to 81, and the average per m. 3 to 228, and remains 

 near this level during the remainder of the year. 



An analysis of the full statistical data, of which the records for 

 1898 are fairly typical, confirms the conclusions drawn from these 

 averages. Chydorus in channel waters is monocyclic, with a well- 

 defined vernal pulse in March- June which includes 95 per cent, of 

 the total annual Chydorus population. There are suggestions of an 

 autumnal pulse, but the data are not sufficient to delimit it. There 

 is no satisfactory evidence that there are recurrent cycles or pulses 

 at briefer intervals during the year. 



The dominating effect of temperature as a regulating factor in 

 delimiting the seasonal distribution of Chydorus is very evident. 

 This, in addition to its appearance in the annual curve of occurrences, 

 is also exhibited most clearly in a comparison of the vernal pulses 

 in the two years of fullest representation in our records, 1896 and 

 1898. The following table gives the data of dates, temperatures 

 of surface waters, and numbers of Chydorus. 



From these facts it appears that the late spring of 1898 delayed 

 the vernal pulse of Chydorus, and that the early spring of 1896 

 accelerated it in that year so that their apices (April 29 and May 24) 

 are four weeks removed from each other in seasonal location. In 

 both years the rapid rise in the pulse appears after 60 is passed, the 

 culmination occurs at about 70, and the decline, in temperatures 

 above 70. 



Egg-bearing females were more abundant during the rise of the 

 pulse, and less numerous during its decline. Evidence of great 

 mortality during the decline of the pulses is to be found in great 

 increase in the relative numbers of empty carapaces. Thus, during 

 the decline of the vernal pulse in 1896 there were on the day of 

 culmination, April 29, 2,780 dead to 18,904 living, on May 1, 3,570 

 to 14,875, and on May 8, 1,578 to 6,706. From 14 to 24 per cent, of 

 the Chydorus population had thus recently perished. Parasitized 



