432 Birge—Vertical Distribution of Pelagic Crustacea. 
and it was desirable to pass from a region containing fewer 
Crustacea to one containing a greater number, rather than the 
reverse. Any accident in washing out the dredge, by which a 
few Crustacea remained adherent to it would introduce a consi¬ 
derable error in passing from the higher to the lower levels, 
while in passing in the opposite direction the error would prac¬ 
tically amount to nothing. Great care was taken in washing 
the dredge, and it is not probable that any appreciable number 
remained adherent to it, but this method was followed to avoid 
any possible error from this source. 
METHODS OF COUNTING AND COMPUTING THE CRUSTACEA. 
The counting and computing were done almost entirely by 
Messrs. Harder and Olson. 
The apparatus employed for counting consisted of a 25 cc. 
glass cylinder graduated to ^ cc., a 2 cc. pipette, graduated to 
At cc., microscope, shallow crystallizing dishes, troughs, and 
registering cards. 
The trough was made of a microscope slide, to which were 
cemented four strips of glass by means of marine glue. Thus 
there was formed a narrow and shallow trough, 60 mm. X 6.5 mm. 
X 2 mm., holding about .8cc. The width of the trough was a. 
little less than the diameter of the field of the microscope when 
a low power objective and eye piece were used. In our counting 
a Leitz objective No. 1 and ocular I were employed. A card 
like the accompanying cut was used in recording the kinds and 
number of Crustacea, as well as the level in which they were- 
obtained, together with other important observations. The 
method used in counting was simple, and with practice became 
somewhat rapid. The catch to be counted was placed in the 
graduated cylinder, and sufficient alcohol added to make 24 cc. 
This quantity was then thoroughly stirred and shaken, so that 
the animals and plants were evenly distributed through it. From 
this mixture two quantities of 2 cc. each were taken and trans¬ 
ferred to watch glasses. One of these quantities was counted 
by each of the observers who were at work. From the watch glass 
there was taken and placed in the trough a sufficient amount of 
material to give about 60 or 70 animals of the most numerous- 
