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. bakeri,axuA 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. 



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 Ploima, 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 



