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). The periods 

 of greatest growth thus lie above 60 and the optimum is 

 near 80. None of these pulses coincides with a volumetric 

 maximum of the silk-net catches (Pt. I., PL XII.). 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 



