247 



in this lake but exhibits great differences in its seasonal course from 

 year to year. The vernal development in May-June (the only one 

 in our channel plankton) is relatively large in each year, but is 

 sometimes exceeded by an autumnal one in October. A midsummer 

 minimum sometimes appears between these pulses, and a winter 

 minimum in December- April is always present. 



From the data here reviewed it seems probable that the very 

 limited seasonal distribution and irregular annual recurrence of 

 D. hyalina in our channel plankton is in a measure indicated in 

 streams elsewhere, and may have its cause in the instability of the 

 fluviatile environment as compared with the lacustrine, where the 

 species evidently finds its environmental optimum. 



Diaphanosoma brachyurum (Lievin). Average number, 479, of 

 which 154 are females, 49 females with eggs, and 276 immature. 



This species in our waters is monocyclic, with sharply defined 

 seasonal distribution. With the exception of two records of young 

 individuals in March- April, 1895 (and the identification of these 

 individuals is questionable), all our records of occurrence in 

 1894-1899 fall between May 25 and October 19, the first vernal 

 records appearing at temperatures of 55.8 to 72.3, and the last 

 autumnal at 52.5 to 65. The one pulse in each year except in 

 1894, when none was recorded falls in a period of 3-6 weeks in 

 July-September, the first record above 2,000 per m. 3 appearing 

 July 26, and the latest (with one exception, 2,175 on September 27, 

 1895) on September 7. The pulse varies in duration in different 

 years from 3 to 6 weeks, and attains a maximum on dates ranging 

 from July 26 to August 31, and varying in amplitude from 8,580 

 to 19,602 per m. 3 An analysis of the distribution of 61 recorded 

 occurrences in channel plankton shows that of these only 13, or 21 

 per cent., occur outside of July-September, and that the records 

 outside of the seven weeks of the pulse include less than 12 per cent, 

 of the total individuals. 



A comparison of the seasonal curve of distribution with the 

 annual thermograph reveals the fact that the pulse occurs toward 

 the close of the period of maximum summer heat, and in every case 

 at a temperature of 78 or above, and that the decline of the pulse 

 often begins with declining temperatures, and is always accom- 

 plished during the autumnal decline. The effect of summer heat 

 pulses upon the Diaphanosoma curve is strongly suggested by the 



