78 



does not directly contribute much, even at its maximum develop- 

 ment, to the volume of the plankton taken in the silk net. On 

 the other hand, its rapid multiplication, as evidenced by its meteoric 

 pulses, may serve to build up a more permanent and bulkier animal 

 plankton, and thus indirectly, in a cumulative way, it may be of 

 considerable quantitative importance. 



The inclusion of all the variants of Dinobryon as a single species 

 has been favored by Wesenberg-Lund ('00), who regards D. stipi- 

 tatum as the summer form of D. sertularia. In our plankton, D. 

 stipitatum has occurred sporadically in December and March, but 

 it is most abundant during the vernal pulse in April-May. Its 

 distribution thus in the main supports that author's contention in 

 that it is found during the warmer portion of the seasonal cycle of 

 Dinobryon in our waters, though not in our summer plankton. It is 

 not desirable in this connection to enter further into a discussion of 

 problems which have been raised by the splitting up of Dinobryon 

 into so large a number of forms. Lemmermann has found seven- 

 teen species and varieties within the limits of the subgenus Eudino- 

 bryon. A discussion of their validity involves not only some per- 

 plexing problems of synonymy, but also an extensive examination 

 of a large amount of material showing seasonal changes, and, 

 above all, a series of experiments which shall demonstrate the 

 limits of variation within a known line of descent and in the sea- 

 sonal range of environmental conditions. It involves, moreover, 

 the fundamental question of the criterion of species. The papers 

 of Lemmermann ('00) and Brunnthaler ('01) have appeared since 

 my work of enumeration was completed. I recognize among the 

 forms which they have sought to establish the following which 

 occur in our plankton: D. sertularia Ehrbg., D. sertularia var. 

 thyrsoideum (Chodat) Lemm., D. sertularia var. alpinum Imhof, 

 D. protuberans Lemm., D. sociale Ehrbg., D. stipitatum Stein, D. 

 stipitatum var. americanum Brunn., D. stipitatum var. bavaricum 

 (Imhof) Zach., D. elongatum Imhof, D. elongatum var. undulatum 

 Lemm., D. cylindricum Imhof, D. cylindricum var. palustre Lemm., 

 D. cylindricum var. schauinslandii (Lemm.) Lemm., D. cylindri- 

 cum var. ped^forme (Lemm.) Lemm., D. cylindricum var. diver gens 

 (Imhof) Lemm., andD. cylindricum var. angulatum (Seligo) Lemm. 



As a result of my attempts to refer all of the individuals which I 

 have seen in my work of enumeration to species, I am of the opinion 



