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 le 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 Pl. 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. 
cartnata. 
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 
(700) 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 
er i ial 
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