85 



of this species is noticeable. Its optimum temperatures lie close to 

 the summer maximum (80), and though perennial, its occurrences 

 at other seasons than late spring and summer are irregular and its 

 numbers few. Its seasonal distribution in German- lakes, as re- 

 ported by Apstein ('96), is similar to that in the Illinois River. 

 The Peridiniidas play but an insignificant part in the plankton of the 

 Illinois River. 



Phacus longicaudus Ehrbg.* Average number, 61,153; silk, 3 ,03 1 . 

 This species in 1898 made its first appearance in the plankton on March 

 23 and continued till November 15. The species is small enough to 

 escape through the silk net, and the data from such collections do 

 not fully express its seasonal fluctuations. There is no marked 

 vernal pulse, and there are traces of but a few small ones during the 

 summer, the largest in 1898 being one of 35,200 on September 27. 

 The distribution in previous years is much the same. A well-sus- 

 tained development throughout the warmer months save when 

 rising floods, as that of May, 1898, reduce the numbers indicates 

 that the optimum temperature for the species approaches the 

 summer maximum (80). There are almost no occurrences below 

 45. This is the most abundant member of the genus in our plank- 

 ton, but it is not quantitatively an important element therein. 



Phacus pleur one ctes Nitzsch.* Average number, 450,000; silk, 

 298. It is less abundant (from one fifth to one tenth) than P. 

 longicauda in the catches of the silk net but apparently much more 

 abundant in the filter-paper collections, which may be due in part 

 to its smaller size and greater tendency to escape through the silk 

 in the collections of the net. Its occurrences are even more closely 

 limited to summer months from June till September. There is no 

 vernal development, and the largest numbers occur during the 

 period of maximum heat. Pulses are but feebly defined. It is also 

 a summer planktont. 



Phacus pyrum Ehrbg. was found but once on August 10, 1897. 



Phacus triqueter Ehrbg. occurred in small numbers during July 

 and August, 1897. 



Platydorina caudata Kofoid. Average number, 17. In 1898 

 this interesting new genus of the Volvo cidoe was found in the plank- 

 ton only in the latter part of July. In 1897 it was much more abun- 

 dant (average number, 21,963) and ranged from July 14 to October 



