213 
these exceptions no record exceeding 600 per m.* was made between 
the dates named. Between April 20 and October 1 the minimum 
records rarely fall below 600 per m.°, except in 1898, and the pulses 
often culminate at 2,000-8,000. C. viridis var. insectus is thus a 
planktont of the warmer season, and its seasonal distribution is 
strikingly similar to that of the so-called var. brevispinosus. 
This form occurs in our plankton throughout the whole seasonal 
range in temperatures, but only in small numbers and irregularly 
below 60°. Only 21 per cent. of the collections containing 
insectus were made at temperatures below 60°, and these contained 
less than 3 per cent. of the total individuals. With the exceptions 
of the pulses culminating at 43° November 23, 1897, at 1,200 per 
m.*, and at 57° April 26, 1898, at 4,160 per m.’, no development of 
this species exceeding 600 per m.* occurs below 60°. All pulses of 
more than 3,000 per m.’, excepting only the April pulse of 1898, 
occur at temperatures above 70°. The species reaches its greatest 
development in channel waters during the period of maximum | 
temperatures, 70°—-80°. 
The seasonal distribution of this form shows a few straggling 
individuals in November—March during temperatures below 50°, 
and a meteoric rise to a vernal pulse in April-May as this tempera- 
ture is passed and 60°—-70° arrives. This is followed by a series of 
recurrent pulses, often of considerable amplitude, through Septem- 
ber or until temperatures fall below 60°, as in October, 1897. With 
falling temperatures the drop in numbers to the winter minimum 
is quickly accomplished. A comparison of the distribution in 1897 
and in other years, shows a close correlation between the decline in 
temperatures and the falling off in numbers of zusectus. 
The relations which hydrographic conditions bear to the develop- 
ment of imsectus in channel plankton may be inferred from the 
hydrographs on Plates [X.—XII, Part I.,and from the data sum- 
marized in the following table,—1894 being omitted because of the 
incompleteness of the seasonal representation. 
In 1895 levels were low, unusually so in the spring, and the 
flood-free intervals of the year were of more than the usual extent. 
About 10 feet of the total movement in levels (51.9 ft.) is found in 
the late December rise. If this is excluded, the total movement 
falls to 42 feet, and the range in levels to 6.5 feet. Under conditions, 
