143 



Rotifer spp. Average number, 199. Some bdelloid rotifers un- 

 identified because of the state of their contraction, or not even 

 questionably referable to other species listed, are here included. It 

 is quite probable that some individuals belonging to the genera 

 Philodina and Callidina are among the number. The occurrences 

 are irregular. They exhibit a distribution with respect to years 

 similar to that noted in the two species just discussed. Vernal pulses 

 are noticeable in 1896 on April 29 (19,446), on April 27, 1897 (28,800), 

 and May 3, 1898 (3,200). Egg-bearing females were_noted in the 

 winter months of 1899, in December and March of the preceding win- 

 ter, and in April, 1896. Individuals parasitized by Endophrys 

 rotatoriorum Przesm. (?) were noted in April, 1896. 



Rotifer tardus Ehrbg. Average number, 6,688. This is the most 

 abundant of all the bdelloid rotifers in our plankton, outnumbering 

 all the others in 1898 six to one. This was due to a sporadic and un- 

 usual pulse of individuals in the plankton in midwinter under the ice 

 in 1898. Owing to this, the average number in 1898 exceeds that in 

 previous years. If, however, the large numbers in January and Febru- 

 ary, 1898, be reduced to normal winter proportions no record in 

 1896 in this season exceeds 7,000 the average for the year falls to 

 about 3,500. The average of occurrences in the plankton for 1896, 

 1897, and 1898 would then be 5,201, 1,254, and 3,500, which approxi- 

 mates somewhat the ratios of the relative disturbance of the hydro- 

 graph in these years (Pt. I., PI. X.-XIL). The agency of flood 

 water in affecting the numbers of this species in the plankton is to 

 some extent indicated by this ratio. It is also apparent on compari- 

 son of the seasonal distribution (Table I.) with the hydrograph for 

 1898 (Pt. I., PI. XII.). The large numbers of January, February, and 

 March appear in every case with rapidly rising water, and the same 

 is true of the numbers on August 9 (12,000) and September 13 (17,- 

 500). Other disturbances than those due to floods, or other factors 

 than disturbances in the water, must be invoked to explain such in- 

 creases as one to 12,800 in April-May, 1898 (Table I.). This attends 

 the vernal volumetric pulse (Pt. I., PI. XII.), but does not conform 

 to its proportions. It appears in the more stable conditions of de- 

 clining flood, and no adventitious factor is apparent to account for its 

 development to such numbers in the plankton. The winter pulse 

 was attended by large numbers of ovigerous females, but none 

 was recorded during this vernal pulse. A somewhat similar increase 



