236 University of California Publications in Zoology (Vou. 14 
tween the occurrence of certain species and the distribution of a par- 
ticular type of bottom is to be expected. 
Information regarding the character of the bottom was obtained 
by the Survey from four sources. The least accurate of these con- 
sisted of taking notes at the time the haul was made of the content of 
the dredge. Often this procedure was attended with difficulty, espec- 
ially when the dredge contained materials of a composite nature. 
This method is, of course, inaccurate if the bottom is diversified. A 
considerable portion of the bottom from a definitely known locality, 
together with the life upon it, was secured by means of the orange- 
peel bucket dredge. The other two methods were even more satisfac- 
tory. Stratified sections, in one case nearly 170 centimeters long, were 
obtained by means of the ‘‘Albatross’’ bottom sampling apparatus 
and the Ekman tubular bottom sampler. 
Physical and partial chemical analyses have been made of such 
tubular samples for 79 stations within San Francisco Bay. An in- 
structive chart has been made by Sumner e¢ al. (1914, pl. 5), on which 
by a system of shaded sectors of circles the proportions of the different 
types of sediments are shown for these stations on an outline map of 
the bay. These data have been made the basis for the discussions that 
follow. 
The data obtained at those stations only at which the bottom con- 
ditions are definitely known are considered in this section. In a few 
instances the position of a dredging station at which a sample was 
obtained was lacking. In such a ease it is approximately the same as 
that of a hydrographic station at which a sample was taken. The 
position of the dredging and the hydrographic stations were declared 
identical, therefore, if a part or all of the dredging course was within 
0.3 of a nautical mile from the latter. This distance was chosen for 
convenience. It represents the approximate equivalent of the radii of 
the circles used on plate 5, as mentioned above, to designate the char- 
acter of the bottom. This chart when superimposed upon one showing 
the location of the dredging stations readily gives the hydrographic 
equivalent of any desired position. 
These 72 stations at which both bottom samples and faunas were 
obtained may be divided into six groups, representing different types 
of bottom. The terms ‘‘mud’’ and ‘‘sand’’ are used in the same sense 
as used by Sumner et al. (1914, p. 92). Sand includes material that 
will pass through a 2-millimeter sieve, yet does not remain in suspen- 
sion in water for any considerable period. These groups may be 
characterized as follows: 
