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. (7?) 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- 
etapm im tiese years (Pt. f, Pl. X.—XIT.). 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 comparti- 
son of the seasonal distribution (Table I.) with the hydrograph for 
1898 (Pt. I., Pl. 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., Pl. 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 
