322 
BULLETIN OF THE BUREAU OF FISHERIES 
conditions, nor to differences in the intensity of the sunlight shining on the surface. 
Concerning the last possible factor, however, the ship’s log showed that the sky was 
clear when the samples were taken at areas J, L, and R. Whatever conditions gave 
rise to the vertical distribution mentioned during the October cruise of 1915, it seems 
probable that they were peculiar to that cruise, for such a distribution is not even 
suggested from an examination of the data from other cruises. 
Paralia sulcata (Ehr.), a tychopelagic diatom listed by Wolfe and Cunningham 
as Melosira sulcata (Ehr.) Kutz, was widely distributed in Chesapeake Bay during 
1916 and was present at least as late as September in considerable numbers. It was 
found even as far north as area X during every cruise except September in 1916, and 
it was present in waters of a great range of salinities and temperatures. 
The highest counts obtained both at the surface and in deep water were during 
the March and April cruises. As examples, the surface counts at area A were as fol- 
lows: January, 2,600; March, 4,200; April, 7,700, June, 0; July, 100; and Septem- 
ber, 1,000. At 27 meters the counts were: January, 4,900; March, 6,700; April, 
4,800; June, 2,400; July, no count; and September, no count. For area J the sur- 
face counts were the following: January, 500; March, 2,400; April, 1,100; June, 
500; July, 200; and September, 200. At 27 meters: January, 3,900, March, no 
count; April, 15,200; June, 500; July, no count; September, 400. 
Again, as in the case of SJceletonema costatum, the highest counts were found close 
to the mouth of the Potomac River, 15,200 at 27 meters and 15,800 at 26 meters for 
areas J and L, respectively, during April. These counts are about double those of 
any other counts during the year for this species. 
This diatom, which has a comparatively heavy test, has the vertical distribution 
of a typical bottom form. The counts of Paralia sulcata showed the highest numbers, 
with one exception, at the bottom; the numbers gradually decreasing toward the 
surface. Data from other cruises, although limited to a surface and a deep-water 
count, showed the highest numbers in deep water with only a few exceptions — namely, 
in very shallow regions and during the spring maximum, when the water is in an 
unstable condition. 
Table 13 gives the counts for Paralia sulcata found in the same samples from 
which the counts for SJceletonema costatum tabulated above were made, so the salin- 
ity, temperature, and other environmental conditions were the same for both. 
Table 13 . — Paralia sulcata, October, 1915 
Meters 
A 
J 
L 
R 
Meters 
A 
J 
L 
R 
0 
700 
500 
700 
27 
800 
1, 200 
2, 100 
1, 600 
4, 700 
3.400 
1.400 
4,800 
q 
500 
300 
36 
1, 400 
4, 800 
18 
700 
3,600 
600 
2,600 
46 
Another common diatom in Chesapeake Bay during 1916 which is not a true 
plankton form is Pleurosigma affine Grun. It was widely distributed through the 
bay from the mouth to the region of Annapolis and occurred in the plankton during 
all the cruises — that is, January, March, April, June, July, and September. In 
addition to this it was present during October and December in 1915. The highest 
counts were found during October, January, and April, and most of these were from 
areas near the mouth of the Potomac River. The highest count for all cruises was 
at area M, in the mouth of the Potomac at 18 meters. The numbers of this diatom 
