140 Dr. C. A. Kof'oid on Pleodorina illinoisensis. 



the same summer the form also occurred in the Illinois River 

 and its adjacent waters (Clinton, 1894), and it has been found 

 in the plankton of these situations in succeeding years from 

 June to September. The distribution of the species in this 

 continent is thus quite extended, and it is not at all im- 

 probable that continued investigation of freshwater plankton 

 will demonstrate that this genus has a cosmopolitan dis- 

 tribution similar to that of some other genera of the family 

 Volvocinese to which it belongs. 



On June 16, 1898, a form which may be referred to the 

 genus Pleodorina was found in the Illinois River in water 

 entering the stream in large part from Cook's Slough and 

 Quiver Lake. Owing to high water (10 feet above low-water 

 mark) prevailing at the time, a considerable portion of the 

 habitat of the form in question consisted of submerged terri- 

 tory, with shallow warm water abounding in growing aquatic 

 and semi-aquatic vegetation. 



This Pleodorina could not be found in Quiver Lake collec- 

 tions made on the 7th of June, but on the 16th it was present 

 in the river in small numbers, increasing until the 20th, when 

 a maximum was reached. From this time the numbers 

 decreased until the 27th, when, following a rise in the river, 

 the species seemingly disappeared entirely from the plankton. 

 It was also found sparingly in Thompson's Lake during this 

 period, a large area of slightly submerged territory being at 

 this time tributary to the lake. 



Associated with this species in great abundance was Eudo- 

 rina elegans in all stages of asexual reproduction, and Pando- 

 rina inorum was also present in smaller numbers and in like 

 condition. Volvox, Euglena, Phacus, Lepocinclis, Trachelo- 

 monas, Dinobryon, Synura, Mallomonas, Uroglena, Melosira, 

 and Fragillaria occurred in varying frequency, but only a 

 single specimen of Pleodorina californica was found in collec- 

 tions containing the species described in this paper. The 

 animal plankton was represented in the main by rotifers, Poly- 

 arlhra being most abundant, while Synclneta, Euchlanis, 

 Pterodina, Brachionus, and Anurcea were also present. 

 Difjlugia, Codonella, Bosmina, Cyclops, and nauplii complete 

 the list of the more common associates of this Pleodorina in 

 the plankton. 



Pleodorina illinoisensis, sp. n. 



The species here described consists of an ellipsoidal cceno- 

 bium or colony of 32, rarely 16 and still more rarely 64, 



