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 firachionus, namely, variabilis, pala, amphiceros, 

 dorcas, rubens, budapestinensis, duniorbicularis, 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 ta the Wabash system, near 

 Urbana, 111. From the relatively small numbers, the slight 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. bakeri 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 poly cyclic 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 w r as 

 found Sept. 21, 1-897, 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, 



