ANNUAL DISTRIBUTION OF THE ORGANISMS. 35 



appeared by the first of August. There was a rapid increase 

 the last of August, another decrease and a rise again about 

 September 20. It then decreased until the middle of Octo- 

 l>er, and had entirely disappeared in December. In July and 

 August it was perhaps the most noticeable element of the plank- 

 ton, its greater specific gravity causing it to settle first in the 

 centrifuging process. In 1900 it appeared as early as MarcL, 

 but did not commence to increase until well into June, and 

 reached a single maximum about the first of August, then der 

 creased rapidly and disappeared about the first of October. 

 In 1901 it is evident that some untoward circumstances must 

 have prevented its development. 



In the collections made early in July, 1899, Eurycercus was 

 much more abundant over the mud bottom well out in the lake 

 than it was over the stony bottom nearer shore. Late in July 

 the conditions were reversed, that is, Eurycercus was more nu- 

 merous over the stony bottom nearer shore. An attempt was 

 made to correlate this fact with the movements of fish, but not 

 with entire success. It was found that while Eurycercus waa 

 prevalent, it was an important article of food for the sheeps- 

 lieads, and apparently there was some movement of the sheeps- 

 heads corresponding to the change in position of the greatest 

 numbers of Eurycercus, but the observations were not suffi- 

 ciently numerous so that any final statement could be made. 



Chydorus sphaericus O. F. Miiller. 



Chydorus occurs only occasionally in Green lake. In the 

 summer of 1899 none were found. Considerable numbers were 

 found in October of 1900, and it continued in small numbers 

 in the collections until January, but was found in none of the 

 later collections. 



In Lake Winnebago (Plate XIX) it appears, to be a perennial 

 form. In the summer of 1899 it had a sharp maximum early 

 in August. It then declined rapidly and only few were found 

 until July of 1900, when there was a sudden and great increase. 

 A still greater increase occurred about the first of November. 



