168 



PI. XI.) prolonged for a fortnight into September, with river water 

 at or above 80. In 1898, it falls away in numbers more rapidly 

 than the spineless form (Table I.) as temperatures fall in October, 

 though this tendency is less marked in previous years. 



Brachionus angularis, as above stated, seems to be rare in the 

 plankton of our more northerly and cooler American waters. It is 

 also conspicuously absent from plankton of Swiss waters, as reported 

 by Weber ('98) and Burckhardt ('00 and '00a),and from German 

 lakes examined by Apstein ('96), Zacharias ('98), and Seligo ('00), 

 and from Finland waters examined by Stenroos ('98). It was, 

 however, found by Wesenberg-Lund ('98) in Danish waters, and in 

 the Udy River, in Russia, by Skorikow ('97), whose statistical records 

 show it to be the most abundant Brachionus in that stream, and 

 outnumbered among the rotifers only by Synchceta stylata and 

 Polyarthra. Schorler ('00) finds it in the Elbe from April to July 

 and most abundantly in June. Lauterborn ('98) reports it as 

 perennial in the Rhine and poly cyclic, with winter eggs in April, 

 June, August, October, and November. This distribution is much 

 like that in the Illinois River, and will probably be found in tem- 

 perate waters wherever the seasonal cycle is thoroughly examined. 



Brachionus bakeri Ehrbg. Average number of females, including 

 all varieties, 594 ; eggs, 420. The following table, giving the average 

 of each of the varieties in the several years, will serve to indicate 

 their relative abundance, the totals showing the relative abundance 

 of the bakeri group in each year and of each variety in the total of 

 all the collections. 



Though the species is greatly diversified by variation the number 

 of individuals is much less than that of many other plankton rotifers 

 in which variation is much less apparent. 



It will be noted that, the species was apparently more abundant 

 in the earlier years. This is only in part the result of the distribu- 

 tion of the collections, as is shown by the fact that the numbers taken 

 were much larger. Thus in 1898 the largest record is 7,600 ; in 1897 

 there are three occurrences in-excess of this; in 1896, two; in 1895, 

 three; and in 1894, four. The largest occurrence, 122,958, was on 

 June 30, 1894. The largest numbers by far were recorded in 1894, 

 a year of low water in spring. The hydrographic conditions of the 

 following year were somewhat similar, but the development of 

 B. bakeri was much reduced, at least at the time of the collections. 



