200 



first of October, and at irregular intervals and in small numbers in 

 fall and winter months. The distribution in years prior to 1898 

 falls within the limits shown in Table I. In this year the bulk of 

 the occurrences lie between June 21 and August 4, and above 

 77 and 72. The optimum lies near the summer maximum, though 

 occurrences at minimum temperatures in March and December 

 reveal acclimatization to a wide range of temperatures. In this 

 year there are several somewhat irregular pulses, the best-defined 

 of which follow the pulses of chlorophyll-bearing organisms (cf. 

 Table I. and PI. II.) at an interval of one or two weeks. The species 

 was not recorded so frequently in previous years, in some of which 

 also pulses are indicated. These pulses are not consequent upon 

 floods, and the species is apparently not adventitious in the plankton 

 but a normal constituent. Apstein ('96) reports M. capucina as 

 abundant in Dobersdorfer Lake from June to October a seasonal 

 distribution similar to that found in the Illinois River for M. 

 carinata. 



Mastigocerca elongata Gosse was found once March 28, 1899, at 

 38. Hempel ('99) reports it in June in Quiver Lake. 



Mastigocerca mucosa Stokes was taken in August to October, 

 1898, at 82- 62, in small numbers. It is reported by Jennings 

 ('00) as "one of the most abundant of the Rotifera among the 

 vegetation of the shallow parts of Lake Erie," but it was not reported 

 by Hempel ('99) in similar environment about Havana. 



Mastigocerca stylata Gosse was found -in the plankton in small 

 numbers in June and July at temperatures approaching 80. Hempel 

 ('99) reports it also in August. 



In addition to the species of this genus above listed, Hempel ('99) 

 records M. lata Jennings. There are also in our records a considera- 

 ble number of individuals referred to this genus but not specifically 

 identified. Many of these belong to one, or possibly several, very 

 small species. They are most abundant during the summer months, 

 reaching a pulse of 16,800 on June 28. They occur in large numbers 

 in the filter collections (average for 1898, 798 ; filter-paper, 145,384), 

 and, it seems, must escape with ease through the silk net on account 

 of their small size and their active movements. 



A number of species in this genus have been described of late 

 from the fresh-water plankton, but in the present state of the litera- 

 ture of the subject I am not certain to what species these forms 



