90 
95) in the summer plankton of the Great Lakes at temperatures 
15° to 20° below the summer maximum of the Illinois River. 
Trachelomonas acuminata Schmarda.*—Average number, 1,094,- 
615; silk, 873. This species appears in the plankton in April or May 
and continues into October or November. There is no vernal pulse, 
and the data are too irregular to trace the seasonal fluctuations. 
The greater numbers occur during the period of maximum heat. 
Excepting a single occurrence in February, this species has been 
found only above 40°, and its period of continuous appearance from 
May to October lies above 60°. It is evidently a summer plank- 
tont. 
Trachelomonas hispida Stein.*—Average number, 1,002,115; silk, 
1,251. This is a perennial organism, found in every month of the 
year but in larger numbers during. the warmer months. It was 
more abundant than usual in the winter of 1897—98 following the low 
water and unusual development of the previous fall. There are no 
large pulses in 1898, but in 1897 there is indication of a vernal max- 
imum on April 27 and an autumnal one of 85,500,000 on September 
7. The data are too irregular to trace the seasonal fluctuations in 
detail. There is no doubt, however, from the evidence at hand that 
this is a predominantly warm-water planktont similar to the other 
members of the genus. 
Trachelomonas volvocina Ehrbg.*—Average number, 17,672,692; 
silk, 7,162. This is the most abundant species of the genus and is 
found throughout the year in almost every collection. It is most 
abundant from May to October, during the period of maximum 
heat. There are no well-defined vernal or autumnal pulses, but 
recurrent maxima during the summer are to be found in both 1897 
and 1898. There are four such pulses in the former year, and in the 
latter five, as follows: May 17 at 64° (14,400,000), June 21 at 77° 
(147,600,000), July 19 at 84° (86,400,000), August 9 at 83° 
(252,000,000), and October 4 at 71.5°-(11,700,000). “‘Thespemede 
of greatest growth thus lie above 60° and the optimum is 
near 80°. None of thtse pulses coincides with a _ volumetric 
Maximum of the silkenet-catehes (Pt..14 ee Sak They 
usually follow these maxima at intervals of one or two weeks— 
a phenomenon often observed in other synthetic species. It may 
be explained by the decrease in animals which feed upon the organ- 
isms in question. These volumetric pulses are predominantly 
