4 
197 
ber 1, but is continuously present in the winter of 1898-99 from 
December 6 till March 28, when collections ceased. 
Male eggs were recorded but.once—April, 29, 1895—and there 
is no other evidence of the cycles of reproduction beyond the pulses 
in numbers. They suggest a polycyclic habit with major pulses 
in spring and fall. Itis apparent that conditions affect these cycles 
greatly, as is seen, for example, in the contrast between the earlier 
years, with low water in the spring, and the later ones, when high 
water was longer continued. 
This variety, rubens, has not been widely reported in the plank- 
ton. Skorikow (’96) finds it in June in the River Udy, and Kertész 
(’94) reports it from Budapest, while Stenroos (’98) finds it in the 
littoral fauna of Lake Nurmyarvi in Finland, and also in the plank- 
ton in July and August. 
Brachionus urceolarts var. bursarius Barrois and v. Daday.— 
Average number of individuals, 206; of eggs, 33. This is a sum- 
mer variety, and forms but a small part—less than one per cent.— 
_of the total number of individuals referred to the species. 
Brachionus variabilis Hempel.—This species was found but once 
in 1898, but was more abundant 1n former years (see table on oppo- 
site page). The largest development which it attained in the Illinois 
was a pulse of 168,222 on August 15, 1894, at 84°. The largest 
number in subsequent years was 5,200 per m.* on August 8, 1896. 
It may be significant of the connection of this form with the urceo- 
laris-rubens group that the great pulse of 1894 was coincident with 
an unusual development of rubens on that date. 
This species is a Summer form, the earliest record being May 24, 
1898, at 74°, and the latest September 25, 1895, at 73°. Its opti- 
mum temperatures lie near the summer maximum. If this form 
should prove to be merely a spinous variety of B. urceolaris it wall 
afford another illustration of spinous varieties of Brachionus appear- 
ing at high temperatures, in accordance with the hypothesis of 
Wesenberg-Lund (’00). 
In Table I. there is given for 1898 the seasonal distribution of 
the free winter eggs of Brachionus. It will be seen that they occur 
throughout practically the whole year, with some increase after the 
times of the April-May and September pulses. 
Cathypna leontina Turner.—Average number, 47, in 1896, a year 
of disturbed hydrograph; less abundant in previous years, and not 
