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 Peridinide play but an insignificant part in the plankton of the 
Illinois River. 
Phacus longicaudus Ehrbg.*—-Average number, 61,153 ; silk, 3,031. 
This species in 1898 made its first appearance in the planktonon 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 pleuronectes 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 Volvocide 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 
