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.labis 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 

 A r . aurita Ehrbg. from the river, and N. tripus Ehrbg. and N. lacinu- 

 lata Ehrbg. ( = Diaschiza lacinulata Ehrbg.) from the backwaters. 



Ploesoma 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 

 ( '99) 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 

 and 11, 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 Rotifer a, and exceeding in 

 numbers all other species of the group excepting only Synch&ta 

 stylata. It is a 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 



