281 



rise above 31 per cent., the occurrences are irregular, and the num- 

 bers are small. Thus in 1896 and 1898, years of numerous winter 

 and vernal collections, there were but 4 occurrences in each prior to 

 July 1 , and all but one of these was of numbers less than 100 per m. 3 

 Only 12 of the 72 occurrences and 8 per cent, of the total individuals 

 were recorded in the first and less stable half of the years. In 

 July-December numbers rise in feebly outlined pulses which attain 

 at the most 800-2,400 per m. 3 The percentage of collections con- 

 taining the species rises to 33-75 per cent., and in stable autumns 

 such as 1895 and 1897 the occurrences are but little interrupted. In 

 its seasonal distribution in channel waters it is thus largely confined 

 to the last and more stable half of the year. 



Its relationship to hydrographic conditions here suggested also 

 appears in a comparison of the yearly averages given above. The 

 average numbers per m. 3 in 1896 and 1898, 87 and 11, are greatly 

 exceeded by those of 1895 (152) and 1897 (367). The total number 

 recorded in July-December in 1897 is 29 times that in 1898. This 

 well-defined predominance in stable seasons, which appears also in 

 the case of the closely related D. siciloides, exceeds that of the other 

 Entomostraca, and indicates a greater sensitiveness on the part of 

 these species to the deleterious effects of flood waters. The long 

 antennas and great development of the feathering of the caudal 

 stylets afford a large area for the attachment of the silt and debris 

 of flood waters, and accordingly facilitate the destruction or removal 

 of Diaptomus from the plankton more quickly than in the case of 

 Entomostraca in which these processes are less developed as in 

 Cyclops or Bosmina. 



The numbers of individuals are too small to delineate accurately 

 the recurrent pulses which are suggested in the data of distribution. 

 In the autumns of 1895 and 1897, when the occurrences are most 

 continuous, the larger numbers tend to fall at the times of the 

 maxima of pulses of other Entomostraca. There is no marked 

 limitation placed upon this species by the seasonal changes in tem- 

 perature. It is found throughout the seasonal range in tempera- 

 tures, though numbers are slightly smaller in channel waters in 

 November-December. Nevertheless it occurs in considerable num- 

 bers in the backwaters in breeding activity under the ice at mini- 

 mum temperatures in December. 



