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The Ohio Journal of Science [Vol. XIX, No. 8, 



they were at the end of the first year, still it does represent an 

 early stage in the plankton of a rock enclosed pond and for that 

 reason I shall include it. The forms found most frequently are 

 given in Table 5. The figures placed in line with each type 

 represent, first the quantity in 100 liters of water and, following 

 this, the percentage of the total catch which it represents. 

 A glance at the table will show that any species not included 

 were practically negligible so far as numbers are concerned. 



TABLE 4. 



The figures for Dinobryon represent colonies. In addition 

 to these there were a great many separated individuals which 

 are not included in the count. The enormous number of col- 

 onies is interesting in view of the time of year at which they were 

 obtained and the temperature of the water. They are ordi- 

 narily considered to be abundant in the cooler months of the 

 year or, at least in cooler waters. This catch was taken in the 

 middle of July from water of 28° C. 



Discussion. Two prominent features of the association in 

 this pond are (a) the entire absence of fish or any other aquatic 

 vertebrate and (b) the preponderance of its insect population. 

 The absence of fish was primarily due to the fact that none had 

 yet been introduced by man. The large insect population was 

 very probably correlated with the fact that no fish were present, 

 especially so the great abundance of pelagic forms such as 

 Notonecta and Gyrinus. The pond seemed to offer optimum con- 



