168 
Pl. 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 Syncheta stylata and 
Polvarthra. 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 polycyclic, 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 bakers 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. bakert was much reduced, at least at the time of the collections. 
