170 
The reducing effect of the recurrent floods of 1896 may be traced 
in the smaller numbers recorded in this year; and the larger num- 
bers of 1897 may be referred to the more stable conditions then 
prevailing. The very small numbers of 1898 may also be due to 
disturbed hydrographic conditions of that year. The number is 
much smaller than in 1896, when the hydrograph was even more 
disturbed, but in this latter year there was more run-off of 
impounded backwaters during the occurrence of B. bakert,and this 
would tend to favor their appearance in channel waters. 
The occurrences and numbers of this species (as a whole) are 
everywhere somewhat irregular, so that pulses of occurrence are- 
somewhat ill defined. Several such pulses are indicated in 1898, 
and others recur in the records of previous years. As suggested by 
the data of 1898 (Table I.), the several varieties share in these pulses. 
The evidence upon this point is much more striking in other years, 
when numbers are larger. For example, in the following table note 
the pulse of 26,800 on August 23, 1897. 
a) 
& 
8 
: @ F 
Date ‘S < = 3S Ss Total 
is S = gy, S S 
= “S 8 “< x 2 
g S S S > RS 
Ss i = 5 S Ss 
Aug. 10 O 0) Q O O 200 200 
6 c O 1,200 200 600 2,000 5,200 7,400 
PES} O 7,800 1,800 3,400 2-200 11,600 | 26,800 
DS. O 200 400 200 O 1,000 1,800 
In their location these pulses exhibit as a rule the same relation of 
coincidence or sequence to the pulses of chlorophyll-bearing organ- 
isms noted in some other species, and they frequently coincide with 
those of other Plorma, but not always. 
This is perhaps the most variable of the rotifers of the plankton. 
At least its variations affect the fixed processes of the lorica and are 
thus quickly and easily appreciated. The species, in common with 
