74 



of growth; that is, when Dinobryon is multiplying rapidly, though 

 they may appear at any season of its occurrence. 



In the enumeration of Dinobryon five types were recognized, and 

 the individuals were assorted to these "species," viz. : D. sertularia, 

 stipitatum, diver gens, angulatum, and undulatum. Some corrobora- 

 tion of the view that we are dealing with a single variable organism 

 and not with five distinct species may be seen in the coincidence of 

 the seasonal distribution, and of the rise, culmination, and decline 

 of the pulses of the five different forms. 



Since these varieties have such a similar seasonal distribution I 

 shall treat them as a whole, discussing subsequently any individual 

 peculiarities which are noteworthy. The average number of 

 individuals of Dinobryon sertularia, including all its varieties, in 

 1898 was 1,979,785. In 1897 the average was much smaller 

 (79,352) owing to the few collections in the winter, when it is most 

 abundant, and to its suppression in the prolonged low water of the 

 autumn of that year. The relative frequency of these different 

 varieties for I shall treat them as such is shown by the average 

 per cubic meter for the year in 1898, viz.: D. sertularia, 407,602; 

 D. sertularia var. stipitatum, 603,911; D. sertularia var. divergens, 

 866,083; D. sertularia var. angulatum, 101,358; D. sertularia var. 

 undulatum, 831. These figures are only approximate, since colonies 

 containing more than one variety have all been included with the 

 predominant variety in the colony, which is usually sertularia or 

 divergens, consequently angulatum and undulatum are more 

 numerous than indicated by these figures. 



The seasonal distribution of Dinobryon in our waters is well 

 defined, and is sharply limited to the period from November to June. 

 Its earliest recorded appearance was November 8 in 1898, while in 

 1896 and 1897 it was not found until in December. It lingers well 

 into June in 1896 and 1898 the two years in which the spring 

 collections were of sufficient frequency to trace its decline. In 1898 

 the latest record was on June 28. Most of the records after May 

 are irregular and sporadic. It is thus absent from the plankton of 

 the Illinois River from the last of June till November or December. 

 In 1895-1896 there was also a winter interval in which no Dino- 

 bryon was recorded during the December- January flood (Pt. I., PI. 

 IX. and X.). In 1897-1898 a similar interval appears, and con- 

 tinues almost to the end of the slow rise of the flood which culmi- 



