181 



October 17, 1894, at 58. With but two exceptions the species was 

 taken only above 70, and the period of most continuous occurrence 

 and largest numbers is near the summer maximum of 80. The 

 optimum is thus near the summer maximum. This species was 

 never taken in the plankton in large numbers, the greatest being on 

 Sept. 14, 1897 (20,000), at 84. On account of the small numbers 

 .and somewhat irregular occurrences the phenomenon of recurrent 

 pulses is here less apparent than it is in more abundant species. The 

 appended table records the best-defined ones. Thesejpulses share in 

 the general ploiman pulses in only about 50 per cent, of the cases, 

 and the most of them coincide with or follow shortly after the pulses 

 of chlorophyll-bearing organisms. 



PULSES OF BRACHIONUS MOLLIS 



So far as I am aware this species has not been found in other 

 waters than the Illinois River and its adjacent backwaters. Hempel 

 ('99) reports it as most abundant in the marshy environment of 

 Flag Lake. 



Brachionus pala Ehrbg. Average number, including all varie- 

 ties: females, 19,969; eggs, 25,974. The following table gives the 

 average number, in the several years, of the varieties here included, 

 and it will serve to show their relative frequency. 



This is the most abundant species of the genus in our waters, the 

 grand total of all occurrences exceeding 9,000,000. As a whole the 

 species was much more abundant in the stable year 1897 (180,998), 

 and less abundant, all things considered, in the disturbed conditions 

 of 1896 (36,665). As a whole the type form pala is less abundant 

 than amphiceros. It forms but 28 per cent, of the total, as compared 

 with 68 per cent, included in the latter variety. Dorcas forms less 



