283 



m. 3 save in two instances. Occurrences of small numbers continue 

 through July, but from August 1 to October 15 appear the major 

 pulses of the year, attaining an amplitude of 1,000 to 8,800 per m. 3 

 With the decline of temperatures in October, numbers fall to levels 

 below 400 per m. 3 , with one exception (December 14, 1897) at 

 700. The percentage of occurrences is, however, high (41 to 44 

 per cent.) and declines only to 33 per cent, in January. The period 

 of greatest numbers of young thus coincides with that of greatest 

 abundance of adults, and lies at temperatures of 70Vand above, in 

 channel waters. 



The effect of hydrographic changes upon the occurrence of 

 young Diaptomus appears in striking form in the annual averages 

 above quoted. In 1898, a year of sudden changes, the average 

 per m. 3 is only 19, while in the stable conditions of the previous year 

 it is 560. The July-December production in 1897 is 28 times 

 greater than that of 1898. In 1896, a year of recurrent but less 

 sudden floods, the average (158) is less than that of 1895 (336), a 

 more stable year. The great reduction of adults noted in 1898 and 

 1896 is thus paralleled by an even greater reduction of the young. 



Osphranticum labronectum Forbes occurs in the plankton of 

 Quiver Lake in small numbers (see Schacht, '98), and was found 

 once in channel plankton in June, 1896. 



AMPHIPODA. 



Allorchestes dentata (Sm.) Faxon. This is an abundant littoral 

 species found amid vegetation, especially in the vegetation-rich 

 backwaters, such as Quiver Lake. It was not often found in channel 

 plankton, being taken only in the summer of 1895, when the July- 

 August floods carried away the vegetation which had accumulated 

 during the antecedent low water. 



ARACHNIDA. 



ACARINA. 



In vegetation-rich backwaters members of the family Hydrach- 

 nid& were frequently taken, along with other adventitious or- 

 ganisms, with the plankton. In channel waters they are less 

 frequent, and are represented principally by Atax, which is parasitic 



