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, 
stipttatum, divergens, 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 wundulatum are more 
numerous than indicated by these figures. 
The seasonal distribution of Dinobryon in our waters is well 
defined, and 1s 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., Pl. 
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- 
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