23 



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 Chlorophycea as a whole exhibit (PI. 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 Chlorophycecz 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 rectangularis, Actinastrum 

 hantzschii, Raphidium polymorphism, Scenedesmus genuinus, S. obli- 

 quus, Richteriella botryoides, Ophiocytium capitatum, Oocystis naegelii, 

 Ccelastrum cambricum, Oocystis solitaria, and Schroederia setigera. 

 With the exception of Botryococcus braunii and the species of Pedias- 

 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, Pedi- 

 astrum would occupy a place in the front rank of importance in the 

 ChlorophycecB of the plankton numerically as well as quantitatively. 

 As quantitative factors in the ecology of the plankton, Pediastrum, 

 Scenedesmus, Ccdastrum, and Botryococcus take precedence over the 

 smaller, though more numerous, forms, such as Raphidium and 

 Crucigenia. 



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 summer 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. 3 , are frequently equaled or surpassed in our waters. 



