260 



as clearly as the nauplii and immature forms. In 1898 the adults 

 form only 10 per cent, of the total. 



The relation which hydrographic conditions bear to the 

 copepodan population may be inferred in part from the comparison 

 of years given above, and from the following table, in which are 

 given the average number of Copepoda per m. 3 and the total monthly 

 movement in river levels in July-December, 1897 and 1898. 



With a total movement of 1 1.7 ft. in July-December in 1897 and 

 nearly three times as much (30 ft.) in 1898, we find copepodan 

 population falling off to less than one sixth that of the more stable 

 year. 



Of the total Copepoda in our records for 1894-1899, 78 per cent, 

 are nauplii of Cyclops and Diaptomus, 13 per cent, are immature 

 Cyclops, and the remaining 9 per cent, are HarpacticidcB, Diapto- 

 mus, and adult Cyclops. Of the twelve forms, Cyclops viridis var. 

 insectus is the most important quantitatively, and includes one 

 fourth of the total adult copepodan population, exceeding the 

 next in importance, C. viridis var. brevispinosus, by over threefold. 



