144 



in stable conditions was found in March and April, 1896, from 40-72. 

 Temperatures of 50-70 were several weeks earlier than usual this 

 year, but the increase in R. tardus came at lower temperatures than 

 in 1898. 



As above stated, this winter pulse, or, rather, sequence of three pulses 

 (Table I.), culminating January 25 (89,397), February 15 (27,000), 

 and March 15 (19,200), came with floods. No such increases attended 

 the somewhat similar hydrographic conditions (Pt. I., PI. XIII.) of 

 1899 nor the winter flood of 1896. There is nothing in the eviron- 

 mental data to explain this unusual occurrence. An unusually 

 large number of females with eggs still attached to the body were seen 

 in the period from January 21 to April 12. Fifty per cent, were 

 ovigerous, carrying a single egg. Numbers of similar free eggs were 

 also noted. Rapid multiplication of the species at the time of these 

 pulses is thus suggested, and these may be dependent upon favorable 

 conditions of nutrition of whose nature no clue is suggested. The 

 species is in the main adventitious,with insufficient evidence of a par- 

 tially limnetic habit at some seasons. 



The species occurs in almost every one of the plankton collections, 

 and thus throughout the whole range of temperatures and environ- 

 mental conditions. The largest numbers were taken during minimum 

 temperatures under the ice; but large numbers also appear at other 

 seasons, and no temperature optimum is definitely indicated, though 

 in years prior to 1898 the larger numbers and more regular occur- 

 rences are to be found in the period from March to November at 

 temperatures above 50. 



Rotifer vulgaris Schrank. Average number, 275. This species 

 has a seasonal distribution though in smaller numbers and fewer 

 occurrences which corresponds somewhat closely with that of R. 

 neptunius (Table I.). The same factors in the environment are pre- 

 sumably operative in modifying its appearance in the plankton. 



Hempel ('99) finds R. macrurus Schrank, Philodina macrostyla 

 Ehrbg., and Callidina elegans Ehrbg. in the plankton of Quiver Lake, 

 adjacent to the river. 



PLOIMA. 



Average number, 571,611, including eggs, which constitute about 

 30 per cent. These rotifers occur at all seasons and are found in 

 every collection. They are quantitatively the most important order 



