Marsh— Limnetic Crustacea of Green Lake. 223 



the plankton, in many cases, is very largely composed of Crus- 

 tacea. 



It seems to me that my collections clearly show that so far as 

 the Crustacea are concerned, while parallel or successive 

 collections may show great similarity in numbers, they may, in 

 other cases vary within such wide limits as to make plankton 

 determinations unreliable, unless they are made from the aver- 

 age of a very large number of collections. Inasmuch as it is 

 practically impossible to take a sufficiently large number of col- 

 lections, it follows that plankton collections largely made of 

 Crustacea, cannot be taken as giving the exact measure of the 

 productiveness of different bodies of water that some authors 

 would have us think. We may say, indeed, with reasonable 

 certainty, that one lake is much richer than another, but it 

 seems to me very doubtful if we can express their relative pro- 

 ductiveness by any definite numerical ratio. 



LIST OF PAPERS QUOTED. 



Apstein, Carl. '92. Quantitative Plankton Studien in Susswasser. 

 Biol. Centralbl. Bd. 12 pp. 484-512, 608. 



'96. Das Susswasserplankton: Methode und Resultate der 



quantitativen Untersuchungen. Kiel und Leipzig. 



Birge, E. A. '95. Plankton Studies on Lake Mendota. I. 

 The vertical distribution of the pelagic Crustacea during 

 July, 1894. Trans. Wis. Acad., Vol. X, pp. 421-484. 



Fitzgerald, Desmond. '95. The temperature of lakes. Trans. 

 Amer. Soc. of Civil Engineers, Vol. XXXIV., pp. 67-114. 



Prance, R. H. '94. Zur Biologie des Planktons. Biolog. Cen- 

 tralbl. XIV., Band, pp. 34-38. 



Fric, Ant., und Vavra, V. '94. Untersuchungen tiber die 

 Fauna der Gewasser, B$hmens, IV. die Thierwelt des 

 Unterpocernitzer und G-atterschlager Teiches als Resultat 

 der Arbeiten an der ubertragbaren zoologischen Station. 

 Prag. 



Haeckel, Ernst. '90. Planktonic Studien. Jenaische Zeitschrift, 

 Vol. XXV. Trans, in Report U. S. Fish Commission, 1889- 

 91, pp. 565-641. 



