Article V. — Plankton Studies. II. On Pleodorina illinois- 

 ensis, a New Species from the Plankton of the Illinois 

 Hirer. By C. A. Kofoid. 



The genus Pleodorina was discovered in 1893 by Shaw 

 ('94) at Palo Alto, California, and in May of the following- 

 year the species Pleodorina californica, upon which the genus 

 was founded, was detected by Mottier ('94) in water from a 

 shallow stagnant pool near Bloomington, Imliana. During 

 the same summer the form also occurred in the Illinois River 

 and its adjacent waters (Clinton, '94), 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 improb- 

 able that continued investigation of fresh- water plankton will 

 demonstrate that this genus has a cosmopolitan distribution 

 similar to that of some other genera of the family Volvocinece 

 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 (ten 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 mormn was also present in smaller numbers and in like 



