180 
This species has never developed large pulses in the channel 
waters of the Illinois. Hempel’s statement (99) that it is “the 
most abundant species of the genus” can apply only to certain 
collections in vegetation-rich backwaters, for in the river it is sur- 
passed in the totals of occurrences in the’statistical records by eight 
other forms of Brachtonus, namely, variabilis, pala, amphiceros, 
dorcas, rubens, budapestinensts, cluntorbicularis, and tuberculus. I 
found it in very great abundance in the July-August plankton of 
Crystal Lake, a shallow warm pond rich in vegetation,formed by 
damming a small creek tributary to the Wabash system, near 
Urbana, Ill. From the relatively small numbers, the shght ampli- 
tude of the pulses, and their somewhat irregular development I am 
inclined to think that the centers of distribution of this species are 
not in the open water of the river and its backwaters, but more 
in the vegetation of warm, shallow regions such as the margins of our 
bottom-land lakes. It is thus to some extent adventitious in our 
plankton. 
The pulses of this species are relatively so small that they do not 
contribute an appreciable amount to the total ploiman pulses, nor 
do more than 50 per cent. of their number coincide with such general 
pulses, though they are sometimes found during their rise. The 
greater part of them coincide with the pulses of chlorophyll-bearing 
organisms (Pl. I. and II.), suggesting a food relationship. 
This species is one of the best-defined in the genus, though in the 
character of its asymmetry it varies toward B. bakert var. tuberculus 
Turner. It exhibits some variation in the degree of asymmetry, in 
the curvature of the spines, and in the surface markings. The indi- 
cations of pulses suggest a polycyclic habit, but no evidence in the 
way of males, male eggs, or winter eggs was recorded which will 
substantiate the inference. A female carrying a winter egg was 
found Sept. 21, 1897, at the close of the period of occurrence. Fe- 
males with one, two, or three summer eggs were found throughout 
the summer and in somewhat larger numbers during the rise of the 
pulses. 
Brachionus mollis Hempel.—Average number of females, 137; 
of eggs, 10. More abundant in previous years, the average in 1897 
being 1,092 and 277, and in 1896, 428 and 56. 
This likewise is a summer planktont. The earliest record of its 
appearance in the plankton is June 17, 1896, at 76°; and the latest, 

