203 
to var. acuminata, 3,852 to var. jugosa, 7,029 to N. striata in the 
narrower sense, and 1,469 to other varieties, including var.labzs and 
var. scapha. The seasonal distribution of N. striata (sensu strictu) 
and var. jugosa lies within the limits of that of var. acuminata, but 
occurrences are too few to trace their seasonal fluctuations. 
This species is reported by Lauterborn (’94) in the winter 
plankton of the Rhine. He also notes the connecting links between 
N. acuminata, N. striata, and N. labis, and regards them as belonging 
to the same “ Formenkreis.’’ Apstein (’96) reports N. acuminata, 
N.labis,and N. striata in lakes of northern Germany and indicates a 
seasonal distribution which coincides closely with that found for 
these forms in the waters of the Illinois. He also reports a March— 
April maximum and only isolated occurrences in midsummer. 
Forbes (’83) finds the species in the stomachs of young Coregonus 
feeding upon the March plankton of Lake Michigan. Seligo (’00) 
also finds it in the winter plankton of Prussian waters. 
Notommata cyrtopus Gosse was found in the plankton in April 
and September at temperatures above 50°. Hempel (’99) reports 
N. aurita Ehrbg. from the river, and N. tripus Ehrbg.and N.lacinu- 
lata Ehrbg. (=Dzaschiza lacinulata Ehrbg.) from the backwaters. 
Plesoma lenticulare Herrick was found in the plankton of the 
river from September to December, 1896, throughout the whole 
range of temperatures from 75° to the winter minimum. Hempel 
(799) reports it from May to December, but principally in vegetation 
Polyarthra platyptera Ehrbg.—Average number of individuals, 
86,674; of eggs, 52,560. In 1897, 94,653 and 58,235; in 1896, 29,653 
emcell 138: in 1895, 28,947 and 20,074; in 1894, 743 and 217. The 
effect of the stable conditions of 1897 and of the recurrent floods of 
1896 is seen in the larger averages in the former year and in the 
smaller ones in the latter. 
This is one of the most abundant rotifers in our plankton, includ- 
ing, as it does, one seventh of the total Rotifera, and exceeding in 
numbers all other species of the group excepting only Syncheta 
stylata. It isa perennial form, and was recorded in every plankton 
collection but two, and it may have been present then. 
The seasonal distribution of this abundant species is very char- 
acteristic of the form which most, though not all, plankton organ- 
isms exhibit. Two prominent features are (1) a limitation of large 
numbers to the warmer months and (2) a rhythmic occurrence of 
