25 
the previous year (cf. Pl. XI. and XII., Pt. I.), especially at the time 
of the autumnal pulse. This may account for the contrast in the 
two years. The Chlorophycee as a whole exhibit (Pl. I. and II. and 
Table I.) the tendency to form a seasonal curve of recurrent pulses 
at approximately monthly intervals (three to six weeks), which gen- 
erally coincide with those of other chlorophyll-bearing organisms. 
Thirty-three forms of Chlorophycee were recorded, and closer 
inspection of the collections will undoubtedly yield a considerable 
additional number either of closely related, and therefore included, 
species, or of those which occur but occasionally or in small numbers 
in the plankton. 
Numerically the leading species in the order of their importance 
are Scenedesmus quadricauda, Crucigenia rectangularts, Actinastrum 
hantzschit, Raphidium polymorphum, Scenedesmus genwuinus, S. obli- 
quus, Richteriella botryoides, Ophiocytium capitatum, Oocystts naegelit, 
Celastrum cambricum, Oocystis solitaria, and Schroederia setigera. 
With the exception of Botryococcus braunt and the species of Pedzas- 
trum, the remaining forms are both quantitatively and numerically 
of minor importance. The species just named were enumerated only 
in the silk-net collections, and ccenobia rather than individual cells 
were listed. If allowance is made for the loss of small individuals 
through the silk, and for the increase that would follow if individ- 
uals rather than ccenobia were the basis of representation, Pedt- 
astrum would occupy a place in the front rank of importance in the 
Chlorophycee of the plankton numerically as well as quantitatively. 
As quantitative factors in the ecology of the plankton, Pediastrum, 
‘Scenedesmus, Celastrum, and Botryococcus take precedence over the 
smaller, though more numerous, forms, such as Raphidium and 
Crucigenta. 
The group is thus well represented in our plankton both in 
species and individuals. The leading planktonts of the group 
reported in European and other waters in lakes and rivers are here 
represented almost without exception by identical or closely related 
species. Botryococcus alone seems to be less abundant than in 
lakes—at least, according to my own observations, it is much more 
abundant in the simmer plankton of Lake Michigan than in that of 
the Illinois River. The maximum numbers of Pediastrum reported 
by Apstein (’96) for Dobersdorfer See in July, when reduced to 
number per m.?, are frequently equaled or surpassed in our waters. 
